Drugs may be used either for their local or general action, and sometimes for a combination of the two. Thus a solution of opium may be applied to the eye for its local effect in relieving irritation of the conjunctiva. It may be given by the mouth or injected under the skin to relieve pain and induce sleep, though the seat of the pain may be far removed, both from the point of injection and from the alimentary canal; or the opium may be applied in the form of a pessary in uterine disease to relieve pain, both by its local action on the part, and its general action on the system after absorption.

In order to produce their general action drugs may be introduced into the system through the skin, subcutaneous cellular tissue, lungs, mucous membranes, especially that of the alimentary canal, serous membranes and veins. The same drug applied in the same quantity through different channels may have different effects; for not only may slower absorption give rise to difference in the amount present at any time in the blood, as already explained (p. 38), but a reflex effect upon the organism may be produced by the local action of the drug at the place of introduction.