These are remedies which lessen the severity or prevent the return of attacks of certain diseases which tend to recur periodically.

The chief of these are :Cinchona bark and its alkaloids :Quinine. Cinchonine. Quinidine. Cinchonidine.

Arsenic. Salicylic acid. Salicylates. Salicin.

Bebeeru bark and its alkaloid :Bebeerine.

Eucalyptol.

Action

The mode in which antiperiodics act is not at present definitely ascertained, nor indeed is the pathology of the diseases which they prevent. Remittent fever, however, has been shown to depend upon the presence of a spirillum in the blood, and there is considerable evidence for considering that malarious conditions are connected with the presence of a bacillus. The periodical return of the attacks in such diseases would appear, then, to be associated with the growth of successive crops of these protophytes, and the action of antiperiodics might be explained by supposing that they interfere with the development of these pathogenic organisms.

Uses

Quinine and cinchona bark are often regarded as almost specific in the various affections due to malarious poisoning, i.e. intermittent fevers, periodic headaches, neuralgias, etc. In tropical remittent fever of malarious origin, quinine is also the best remedy we possess. It must be given in very large doses, however, and is less certainly curative than in intermittent fever. The other cinchona alkaloids have a similar action to quinine, but are not quite so powerful: they, as also quinine, may be used as prophylactics in order to prevent the recurrence of ague in persons travelling through or living in malarious districts as well as for the purpose of curing malarious conditions already present.

Arsenic is sometimes even more powerful than quinine, but as a rule it answers best in malarious conditions which are sometimes known as masked or latent malaria, and which manifest themselves in neuralgia and nervous or digestive disturbance rather than in well-marked ague fits.

Adjuncts

Emetics and purgatives aid the action of anti-periodics, and sometimes, indeed, can replace them and cure ague without their aid. Antiperiodics rarely succeed if the functions of the liver are disturbed unless they are aided by emetics or purgatives, and especially by cholagogues.