Caffein was discovered in 1820 by Runge,(77) Pelletier,(60) and
Robiquet(75) and was first analyzed by Dumas and Pelletier,(20)
but its exact percentage composition was determined by Pfaff and
Liebig,(71), (72) while to Herzog (13), (18) belongs the credit of
having established that it is basic. Strecker(82) prepared caffein
synthetically by heating theobromin silver and methyl iodid in a
closed tube for 24 hours. Soon after its discovery in coffee Oudry(67)
reported the presence of a substance in tea which he called "thein."
Its identity with caffein was established 15 years later by Jobst(43)
and also by Mulder.(62), (63) According to Brill,(13) Mulder (1838) was
also the first to perform experiments with caffein on animals. After
the administration of one-half grain to a pregnant rabbit he observed
loss of appetite and kyphosis. The rabbit aborted but recovered
from the effects of caffein. It has since been made the subject of
numerous investigations which were carried out on a variety of animals.
Observations with caffein were also made on the human subject. About
four years after Mulder published his results, Lehmann(51) (1842)
reported experiments on a number of people who were given caffein. The
administration of from 2 to 10 grains of the alkaloid was followed
by headache, palpitation of the heart, increased frequency and
irregularity of the pulse, tinnitus aurium, photopsia, insomnia, and
even delirium. Similar experiments reported by Frerichs(25) (1846)
indicate that in doses of 25 grains it may induce severe symptoms about
15 minutes after its administration. He also observed circulatory as
well as nervous symptoms and vomiting.
According to Albers(2) (1852), 4.5 grains of caffein citrate injected
subcutaneously into the thigh of a rabbit was soon followed by
diminished motion and tremors of the operated thigh. Other symptoms
reported were spasms of the facial muscles, increased respiratory
movements, and mental confusion. Of interest in this connection are
the experiments of Cogswell(17) (1852) on frogs. He concluded that in
point of destructive action on the tissues, caffein is far superior to
morphin and may be compared to strychnin and coniin, its action on the
nervous system he believed to be principally confined to the effect on
the brain and spinal cord.
Lehmann(52) (1853) observed increased frequency of heart action after
the administration of 4 grains, which were given with a normal diet
to an adult man. When the dose was doubled the frequency of the pulse
was still more increased, heart action became stronger, and tremors
and confusion of thought with excitement of the imagination made their
appearance. There was also an increased desire to micturate.
Stuhlmann and Falck(83) (1857) were the first to make a study of the
toxicity of caffein on animals of different species. The administration
of 0.5 gram of caffein subcutaneously or per rectum in rabbits induced
tremors, tonic and clonic convulsions, paralysis, and increased
frequency of respiration at first followed by violent dyspnoea. On
autopsy he noticed congestion of the organs and in two of the three
rabbits experimented upon punctiform hemorrhages of the brain with
congestion of the meninges were found. In the other rabbit anemia
of the brain was observed. Experiments on cats were carried out by
subcutaneous, intravenous, and rectal injections. The symptoms observed
after the administration of 0.5 to 0.7 gram of caffein were the same
as in rabbits except that the cats developed diarrhea when caffein
was given and no anatomic lesions were found on autopsy. The effect
of caffein on dogs indicated that in subjects of medium weight a dose
of 0.5 gram given by mouth might produce restlessness and increased
frequency of respiration, while the injection of the same amount
intravenously into such animals may cause death. Large, full-grown
dogs, however, survived an intravenous injection of 2 grams of
caffein, showing symptoms of incoordination, salivation, and frequent
defecation. These investigators also made observations on caffein,
using pigeons and other birds; 0.5 to 0.1 gram introduced into the
stomach caused vomiting, diarrhea, tonic, but more frequently clonic,
convulsions, incoordination, tremors, paresis, and paralysis.
In a few, but not in all of the birds, there was at first increased
frequency of respiration followed by dyspnoea and circulatory
disturbances. These amounts of caffein proved fatal in all of the
experiments on birds. Inflammation of the intestinal mucosa and
congestion of the meninges were the only changes found on autopsy.
Stuhlmann and Falck also studied the effects of caffein on fishes and
toads. Mitscherlich(60) (1858) fed 0.4 gram of caffein with bread to
a rabbit and noticed lowered temperature, fatigue, convulsions, first
increased then decreased frequency of respiration, and on autopsy
congestion of all the viscera. He also reported observations on two
frogs, one of which was given one-sixteenth of a grain of caffein in
a pill with bread. It was administered to the other frog in aqueous
solution, but the mode of administration was not published. The
symptoms observed were in the main the same as in rabbits. In pigeons
0.125 gram introduced into the stomach caused severe vomiting, muscular
incoordination, tonic rigidity of the limbs, and retraction of the
head. Respiration was increased in frequency. Death followed within 3
hours and 15 minutes.
From a series of experiments on frogs which Hoppe(38) carried out
(1858) by applying one-fourth of a grain of caffein to the muscles of
the back, he concluded that caffein causes paralysis of the nerves,
spinal cord, and brain, sensation being paralyzed before movement.
The injurious action of caffein proceeds, according to Hoppe, from
the spinal cord. This was based on experiments on two frogs, _Rana
esculenta_, in which the right leg was amputated, the nerve being left
intact, while the nerve of the other leg was ligated. At the end of 30
minutes paralysis was more marked on the right than on the left side.
In another frog of the same species he resected the femoral nerve on
the right side; about 1½ hours after the administration of caffein
convulsions were observed. The left leg was rigid, but the right was
relaxed.
Voit(85) (1860) ligated the vessels of the right lower extremity, cut
the nerves of the left leg, and introduced a few drops of caffein
solution into the stomach. Shortly afterwards tetanus of the right
leg occurred on touching the back of the animal; the left leg was
motionless. Later the entire body exhibited tetanic convulsions. From
this and similar experiments Voit concluded that caffein acts first and
principally on the central nervous system, and that caffein is also
poisonous to nerve and muscle fibers, as they die when a solution of
caffein is applied to them. The action of caffein, according to Voit,
is similar in great part to that of strychnin. The effect on the blood
vessels is particularly interesting, as Voit observed dilatation of the
vessels, due as he thought to muscular paralysis, and also transudation
and congestion of the capillaries.
Kurzak(48) (1860) made a study of the comparative toxicity of caffein
in frogs and rabbits and came to the conclusion that the lethal dose
for frogs is about one-seventh of that for rabbits. Caffein citrate
in the form of crystals was administered in both cases by mouth. The
doses given to frogs were 1 to 1.5 grains. He observed convulsions and
increased respiratory activity at first; after one hour respiration
diminished and voluntary muscular activity disappeared. Even on the
second day convulsions were sometimes noticed. Death occurred at the
end of the first or second day. Experiments on only two rabbits were
reported, 0.8 gram of caffein citrate causing the death of one at the
end of 13 hours. The symptoms noticed were the same practically as in
frogs, but it is interesting to observe that ecchymosis of the mucous
membranes of the stomach near the cardia was the only lesion found on
autopsy. Several experiments made on different days on the other rabbit
indicated that the toxic dose exceeded 0.5 gram, while smaller doses
caused but very mild symptoms.
According to Gentilhomme(27) (1867), after caffein the reflexes are
at first diminished and then disappear altogether. Death is produced
by stiffness and immobility of all the muscles, particularly of the
muscles of respiration, thus causing asphyxia. He furthermore held
that caffein has no effect on cardiac or smooth muscle fiber, its
action being specific on voluntary muscle fiber, contractions of
which he observed under the microscope, thus differing completely from
strychnin, which is a nerve poison.
These observations seemed to be confirmed by Pratt(74) (1868), who
reported that the isolated posterior extremities and muscle fibers of
the toad placed in a solution of caffein (1 grain to a wineglassful
of water) for three minutes were contracted, while controls placed in
distilled water were relaxed. This experiment is, of course, defective,
as normal salt solution should have been used in both cases. When the
muscular fibers previously immersed in caffein solution were placed
under the microscope violent contractions were observed. The same
author administered from 2 to 18 grains at a dose to five healthy young
men. After the administration of 12 grains he noticed mental anguish,
tremors of the hands and arms, and insomnia. Doses under 5 grains had
no marked effect except a diminution in the frequency of the pulse and
wakefulness.
About the same time Amory(4) (1868) published the results of his
studies on the toxicity of caffein in cats, dogs, rabbits, and pigeons.
In all cases very large doses were introduced directly into the stomach
by means of a temporary gastric fistula. Ten grains given in meat to a
dog caused restlessness, but no other symptoms. Doses of 30 grains and
above were invariably fatal. Seventy-three grains given to a cat caused
death within 20 minutes.
From observations on frogs, guinea pigs, rabbits, and on one dog,
Leven(53) (1868) concluded that caffein which he gave in the form of
the citrate in doses of 10 mg to frogs, from 150 to 200 mg to guinea
pigs, and three to four times the latter amount to rabbits, stimulates
the central nervous system and the voluntary, cardiac, and smooth
muscles. He found that 0.9 gram caffein was fatal for a rabbit when
injected subcutaneously, while 1 gram of the citrate was not toxic
for a dog of medium size. Caffein applied directly to muscle fiber
causes tetanus and destroys muscular contractility, while a nerve fiber
similarly treated loses its irritability.
According to Johansen(44) (1869), caffein acts directly on the muscular
fiber. After the subcutaneous injection of 0.02 gram of caffein
into frogs, he observed contraction of the muscles at the site of
injection, then contraction of the anterior extremities, and finally
the posterior extremities become rigid and extended. Johansen observed
muscular rigidity after caffein, even after curara was injected,
or after ligating the vessels, or cutting the nerves which supply
the muscles. He also observed that large doses of caffein diminish
muscular irritability. When cardiac muscle was poisoned with caffein,
microscopical examination showed that the striations disappeared.
Johansen also states that reflexes disappear after caffein poisoning.
He never observed tetanus in frogs, but reported tonic and clonic
convulsions as a result of caffein poisoning in mammals. Somewhat
different effects of caffein in frogs were observed by Buchheim and
Eisenmenger(14) (1870). After the injection of 2 per cent of the
citrate the frogs soon become inactive. He also observed muscular
twitching of the extremities, which gradually increased, with rigidity
of the muscles and opisthotonos, while respiration became slow and
superficial, finally stopping altogether.
Aubert(6) (1872) studied the toxicity of caffein in man and other
animals. After the ingestion of 0.36 gram, he observed dizziness, but
doses of 0.12 and 0.24 gram were without any apparent effect. On the
other hand, a dose of 0.5 gram of caffein was followed by increased
frequency of the pulse, which soon disappeared. After one hour he
noticed dizziness and trembling of the hands, which likewise passed
away soon. The injection of 0.16 gram of a 2 per cent solution of
caffein into the jugular vein of a rabbit weighing 1,090 grams caused
tetanus and death in two and one-half minutes, and 0.12 gram injected
into a rabbit weighing 980 grams caused death in one minute. Much
larger doses could be borne, however, when artificial respiration was
resorted to. A dog which was given 3 grams of caffein survived when
artificial respiration was performed. Aubert reports, on the other
hand, a similar experiment with 0.25 gram of caffein which terminated
fatally.
That caffein may give rise to different effects in various species
of animals was observed for the first time by Bennett.(9) He studied
its action on frogs, mice, rabbits, and cats, and attempted to
determine the minimum fatal dose in rabbits and cats. He also reported
experiments with thein. In his first communication on the subject he
states that the administration of thein to rabbits first increased
and then diminished the frequency of respiration, while the pulse
was decreased in frequency. Caffein, which he apparently thought was
different from thein, caused increased frequency of respiration, while
the pulse was markedly retarded after a preliminary acceleration.
He also noticed congestion of the ears, muscular incoordination,
tetanus, paralysis, diminished reflexes, and contraction of the pupils.
Bennett reported the minimum fatal dose of caffein for a rabbit
weighing 3.25 pounds as being 5.25-5.5 grains. The symptoms in cats
after the administration of toxic doses of thein or of caffein were
great excitement, paralysis alternating with convulsions, and profuse
salivation. The minimum fatal dose for a cat weighing 5 pounds was,
according to Bennett, 6 grains of caffein and 5.5 grains of thein. Only
one experiment on a mouse is reported; the administration of 0.1 grain
proved fatal. The symptoms were the same as those observed in cats and
rabbits after the administration of caffein. The experiments on frogs
indicate that the symptoms were about the same as those previously
described in the case of warm-blooded animals except that the reflexes
are almost completely lost after the subcutaneous injection of doses
of one-sixteenth to one-twelfth of a grain. The latter dose was fatal
for frogs. It would be of interest to know the comparative toxicity of
caffein to frogs and mammals, but unfortunately the weights were not
reported.
Schmiedeberg(79) (1874) noticed that the administration of 20 mg
of caffein to frogs weighing about 45 grams was followed, in _Rana
esculenta_, in about 25 minutes, by increased reflexes, 7 minutes later
by tetanus. Several attacks occurred, but tonic spasms were never
observed. On the contrary, when the same amount of caffein was given to
_Rana temporaria_ weighing 45 grams he noticed a marked diminution of
the reflexes and tonic rigidity of the muscles after 23 minutes; the
reflexes were greatly increased, however, about 24 hours later. The
frogs were under observation for three days, and although symptoms were
still present at the end of this time in the subjects of both species
tetanus was never observed in _Rana temporaria_.
Peretti's(70) (1875) studies on the effects of caffein were confined
chiefly to observations on dogs. He also made observations on a few
rabbits and reported an experiment on one cat to which he administered,
by subcutaneous injection, 0.18 gram of caffein per kilo and noticed
increased frequency in lachrymation and crying. The cat was found
dead the next day. The subcutaneous injection of a rabbit in which
artificial respiration was instituted with 0.36 gram of caffein per
kilo proved fatal soon after the injection without any manifestation of
symptoms. Small doses of caffein, 0.1 gram, given to a rabbit weighing
3,670 grams, failed to produce any visible effects. Doses under 0.1
gram per kilo likewise failed to induce any symptoms in dogs. When
0.1 gram of caffein per kilo was given by mouth or subcutaneously it
was followed by restlessness, salivation, rigidity of hind legs, and
vomiting. In both instances the dogs recovered. The symptoms were more
severe when the dose was increased to 0.185 gram per kilo, but even
in this case the dog recovered. A dose of 0.2 gram per kilo, however,
proved fatal.
Henneguy(36) (1875) experimented on three frogs to which he gave 0.01
gram of caffein citrate subcutaneously. He observed mild stimulation
of the nervous system and of the muscles, as well as increased cardiac
activity. Later, voluntary movement and respiration disappeared and
sensations diminished, but convulsions of the extremities appeared.
Cardiac activity was then diminished, the heart being finally arrested
in systole. Since the motor nerves retained their irritability even
after the reflexes disappeared, he concluded that the loss of motion
was due to the action of caffein on the nerve centers.
Binz(11) (1878) reported experiments on dogs and also made some
observations on man with caffein. The subcutaneous injections of 0.2
gram caffein may prove fatal to dogs, although some survive such a
dose. The toxic dose in man varies from 0.5 to 1.5 grams. Disturbance
of the circulation, such as palpitation of the heart and fullness of
pulse, restlessness, and diarrhea were the symptoms he observed.
Extensive investigations on the action of caffein were carried out by
Leblond(50) (1883), who studied its effect on the circulation in man
and lower animals, and its toxicity in the lower animals alone. Five
to twenty centigrams of caffein and 0.06 to 0.25 gram of salicylate
of soda were dissolved and injected into the muscles of the thigh
of young guinea pigs weighing a little over 300 grams. In the three
experiments reported the death of the animals occurred after 23
minutes, 40 minutes, and 1 hour and 20 minutes. Symptoms appeared in
from 10 to 15 minutes after the injection of caffein. Incoordination of
movements, convulsions, both tonic and clonic, opisthotonos, tremors,
increased frequency of respiration, ataxia, paralysis were the symptoms
observed. It is worthy of note that the appearance of paresis preceded
the convulsions. Diminished sensation was reported in one pig, but no
sensory disturbances nor reflexes had been observed in the other. Two
rabbits, one of which received 0.5 and the other about 0.3 gram of
caffein per kilo with equal parts of salicylate of soda, were injected
subcutaneously into the thigh. Diminished sensation, paresis of the
posterior extremities, hyperexcitability, convulsions, opisthotonos,
dilation of the veins of the ear were observed. Death followed in 1
hour and 23 minutes in one rabbit and in 3 hours and 7 minutes in the
other.
Filehne(22) (1886) experimented with caffein on _Rana esculenta_ and
_Rana temporaria_. The subcutaneous injection of 7 mg of caffein into
_Rana esculenta_ caused tetanus, while 50 mg given by mouth caused
tonic spasms. He further stated that the difference between _Rana
esculenta_ and _Rana temporaria_ as regards the reaction to caffein was
one of degree only.
Amat(3) (1889) reported experiments on three guinea pigs, in which 0.4
to 0.5 gram per kilo injected subcutaneously proved fatal within 38 and
44 minutes. One guinea pig which received 0.1 gram of caffein per kilo
survived. The symptoms observed in the two fatal cases were general
muscular rigidity and convulsions.
Parisot(68) (1890) made a study of the toxicity of caffein on different
species of animals. Unlike most of his predecessors, however, he
reported, at least in some cases, the weight of the animals on which
he worked. After the subcutaneous and intramuscular injections of from
5 to 20 mg of caffein into _Rana temporaria_ weighing from 14 to 16
grams, he noticed increased irritability at first; later, a loss of
reflexes, inability to use the muscles, complete muscular rigidity
resembling rigor mortis, and also cessation of heart action. The effect
of caffein produced in the green frog was analogous to that observed
in strychnin poisoning. Parisot found, however, that muscular rigidity
developed, although very gradually, also in the green frog, but it
set in much later than in frogs of the other species and without
superseding the clonic convulsions. According to Parisot, the muscular
rigidity after caffein persists after the destruction of the brain and
spinal cord, thus showing that it is not of nervous origin. He further
emphasized the difference in the behavior of these two species of frogs
toward caffein by stating that he never observed tetanic convulsions
in the red frog. His experiments also indicate that the green frog is
more resistant to caffein than _Rana temporaria_, as the same doses
which are fatal for the latter were only toxic for _Rana esculenta_.
The number of experiments, however, is too few to justify a positive
conclusion on this point. Parisot also made some experiments on
turtles. The results he obtained show that caffein is at least as toxic
for these animals as for the frogs he experimented upon, 0.33 gram per
kilo (carapace not included in weight) having proved fatal within 24
hours. Two experiments on one pigeon were also reported by the same
observer; two doses of 0.06 gram per kilo given at an interval of four
hours caused mental depression and muscular rigidity, but the pigeon
survived.
Experiments with caffein on the human subject made by Parisot showed
that man is far more susceptible to this substance than the other
animals he investigated. After the ingestion of 0.3 gram of caffein
symptoms of intoxication pointing to cerebral disturbance appeared,
which became more marked when the size of the doses was increased.
It will be noticed that the nature of the action of caffein, whether
it is a nerve or a muscle poison, formed the subject of several
investigations. Binz(11) (1890) brought forward additional evidence
in support of the view that caffein acts primarily on the ganglion
cells, and not on the muscle directly. This he has shown by injecting
0.5 gram into each of two rabbits after cutting the sciatic nerve
on one side; in one case he also resected the obdurator and crural
nerves on the same side. Clonic spasms developed in both subjects soon
after caffein was given, but in each rabbit the side operated upon
remained paralyzed. Baldi(8) (1891) studied the action of caffein on
_Rana esculenta_. After injecting from 4 to 20 mg tetanus, such as
observed in strychnin poisoning, was noticed. Fröhner(26) (1892) made
observations on the comparative toxicity of caffein in domesticated
animals. After the administration of 5 grams of caffein sodium
salicylate by mouth to a dog weighing 10 kilos, he noticed salivation,
restlessness, vomiting, and convulsions as in strychnin poisoning.
Death occurred three hours after the drug was given. On autopsy he
noticed mild inflammation of the mucous membranes of the stomach and
intestines and edema of the lungs; the heart was in diastole. A dose
of 2 grams of caffein sodium salicylate given to the same animal
subcutaneously two days previously provoked only very slight symptoms.
The subcutaneous injection of 10 grams of the same preparation into
a pig weighing 30 kilos caused death in two and a half hours, with
the production of symptoms of disturbance of the nervous system and
of gastrointestinal irritation. The same dose per kilo of body weight
given to a goat likewise caused death in two and a half hours after its
administration. Examination on autopsy revealed inflammation of the
gastrointestinal tract. Similar lesions were found in a horse killed by
100 grams of caffein, in which he also noticed hemorrhage of the mucosa
in the fundus of the stomach.
Gourewitch(28) (1907) conducted experiments with caffein on rabbits,
pigeons, and white rats. It appears from his protocol that single doses
of about 0.2 to 0.25 gram caffein per kilo given subcutaneously proved
to be fatal. He states, however, that the resistance to caffein was
markedly diminished, when its administration was repeated daily, for
much smaller amounts sufficed to cause death in these animals. A dose
of 120 mg of caffein per kilo proved fatal after the third injection.
When the dose was increased to 170 mg per kilo, the animal succumbed to
the effects of caffein after the second injection. His experiments on
the other animals do not indicate the degree of resistance to caffein,
since the weights for some were not given while for the others no
attempt was made to determine the minimum toxic or fatal dose.
Maurel(55) (1907) studied the influence of different methods of
administration on the toxicity of caffein on frogs and rabbits. He
determined the minimum toxic and lethal doses of caffein hydrobromid
which he employed in 1 to 2 per cent solutions. He concluded from his
experiments that the toxicity of caffein when given by mouth is twice
as great for the frog as for the rabbit.
More recently Hale(33) carried out a number of experiments on guinea
pigs in which he determined the toxicity of caffein given in the
form of the citrate and made into a pill with mucilage of acacia and
arrow-root starch. After the pill was dried it was fed to the animal,
due precaution being taken that none of it was lost during feeding.
From experiments on guinea pigs which received doses of 0.3 to 0.6 gram
caffein citrate, the following data have been reported: Three decigrams
per kilo given to one pig was not fatal. Of three pigs which received
0.4 per kilo, one died and two survived. Exactly the same results were
obtained in three others which received 0.5 per kilo. Two guinea pigs,
which received 0.55 and 0.6 per kilo each, died after 15 and 7 hours,
respectively, while another animal survived a dose of 0.45 per kilo.
This review of the literature on the toxicity of caffein, although
bearing evidence of considerable investigation and extending over
three-quarters of a century, is largely qualitative in character. It
appears from the experiments that the main object of the investigations
was to ascertain the nature of the action of caffein, whether it is a
muscle or a nerve poison. The comparative toxicity in different species
of animals by the accurate determination of the toxic and fatal doses
received but little attention. To fill the gap in our knowledge of the
toxic effects of caffein, the present investigation was undertaken.
This, it will be seen, proved to be a most laborious task, because
in the large number of experiments careful observations showed that
individuals of the same species varied considerably in their reaction
to the drug. Numerous other factors, as will be shown, were also found
to play an important part in the determination of the toxicity of
caffein.