Quinine is an alkaloid having the chemical formula . Along with cinchonidine, cinchonine, and a large number of other alkaloids, it is present in the bark of numerous species of Cinchona and Remijia, of which these substances constitute the active medicinal principles. Good barks yield an average of 5 to 6 per cent. total alkaloids, of which one-half is quinine and cinchonidine, the other half consisting of the other alkaloids in varying proportions. Quinine is by far the most important from a medical and commercial point of view; the yield of it varies greatly, and 8 per cent. being the extremes. Quinine is obtained from the powdered bark by treating it with lime, and then extracting the mixture with alcohol, neutralising with an acid so as to obtain a salt of quinine, and finally purifying the product. In 1820 Pelletier and Caventou isolated pure quinine, and demonstrated that it was the chief active ingredient in the bark. Many attempts have since been made to prepare it artificially, but without success. For the introduction of the bark into Europe, and the culture of the tree in South America and (recently) in India, see CINCHONA.
Quinine itself is not used in medicine, owing to the inconveniences arising from its insolubility in water, but many of its salts are, and two of them, the sulphate and hydrochlorate, are included in the British Pharmacopœia along with numerous preparations of cinchona-bark containing them. The sulphate is the most commonly used preparation, and it is popularly known as quinine. It occurs in small, silky, snow-white crystals, which have a purely and intensely bitter taste, and are sparingly soluble in water (1 in 700 parts); its solutions have a bluish or fluorescent colour even when very dilute. In alcohol or dilute sulphuric acid it is very soluble. The hydrochlorate closely resembles the sulphate, but is much more soluble in water (1 in 34 parts), and its solutions are not fluorescent. When treated with excess of chlorine water and a few drops of ammonia solution, solutions of quinine give a clear emerald green colour; if ferrocyanide of potassium be added this changes to a ruby red.
Preparations of quinine, and especially the sulphate, are very largely used in medicine. Locally applied dilute solutions (2 to 4 gr. to the oz. of water) have a germicidal, antiputrefactive, and antifermentative action, hence they are used as lotions in hay fever, diphtheria, cystitis, and similar diseases. As a bitter tonic small doses ( to 2 grains) are frequently given in general debility, atonic dyspepsia, anaemia, scrofula, convalescence from acute diseases, and other conditions where tonic treatment is required. It is also of great value as an antipyretic and antiperiodic. In healthy persons it does not reduce the bodily temperature, but in typhus, typhoid, rheumatic, and some other fevers it is extremely valuable in this respect. A dose of 3 to 15 grains may be given in these cases. In certain fevers it does not reduce temperature. In malarial affections of all kinds it is supreme, and at present no other known drug can compare with it in efficacy. In intermittent fevers and ague the best plan is to begin its administration about eight hours before the attack is expected, and continue it in hourly doses for three or four hours until 15 to 30 grains have been given. It cuts short or aborts the recurring febrile attacks. It is also of great value as a prophylactic in persons who are exposed to the risks of malarial poisoning. In such cases three grains twice daily is usually considered a sufficient dose.
Quinine is also largely given in neuralgia and in inflammations. Large doses are very apt to irritate the stomach, and sometimes produce a train of symptoms known as cinchonism. There is singing in the ears, dizziness, deafness, a feeling of fullness in the head, and disturbance of vision—all of which usually pass off without leaving any ill results. In some cases the effects are more severe, and may lead to dangerous depression and collapse, especially if the patient be already weakened by disease. Certain persons are very susceptible to the action of quinine, and suffer from cinchonism after small doses. In others skin eruptions, œdema of the face, irritation of the bladder and kidneys, and other disagreeable effects are sometimes seen. Workers in quinine-factories also occasionally suffer from skin eruptions. These accidents are, however, comparatively rare.
The sulphate of cinchonidine and sulphate of cinchonine are also included in the British Pharmacopœia. They have the same actions and uses as quinine, but are very much less used. They seem, however, to be efficacious, and are cheaper.
See works by J. E. Howard (1876), C. R. Markham (1880), Manson (1882), and Flückiger (1884).