As might be surmised, the same dose of a drug will exert its usual form of activity more easily if given with other drugs of the same class; and sometimes a combination of two similar drugs will gain a result that one alone will not give in any dose. Drugs which help each other in this way are known as synergists, or mutual helpers, and examples are bromides and chloral hydrate for sleep, calomel and jalap for catharsis.

On the contrary, a drug may lose part or all of its power because of some agent that has the opposite physiologic effect. Such opposing agents are known as antagonists. An antagonist may be a drug, or it may be a substance formed in the body, as epinephrine or thyroiodine or some antitoxin. The antagonists may act - (a) on the same structures - for example, bromides prevent the convulsions of strychnine, both acting on the spinal cord; caffeine stimulates the psychic and motor centers of the cerebrum, while alcohol depresses them; pilocarpine stimulates the vagus nerve-endings, which are depressed by atropine; (b) on different structures - for instance, digitalis slows the heart by stimulating the vagus center, while atropine prevents this effect by depressing the vagus nerve-endings; adrenaline stimulates the nerve-endings in arterial muscle, causing contraction of the arteries, and this effect can be wholly neutralized by nitroglycerin, which depresses the arterial muscle itself.

Incompatibility should not be confused with antagonism. It is a pharmaceutic term, and should be confined to prescriptions. Incompatibility may be said to exist between two substances when their admixture in a prescription results in chemic or physical change (other than mere solution). Examples are the precipitation when strychnine sulphate in solution comes in contact with tannic acid, or when lead acetate solution is mixed with a solution of alum. Such a change may or may not be desired in a prescription; hence the physician should know what changes may take place in substances likely to be prescribed together. (See Chapter on Prescriptions.)