Antihistamines appear to compete with histamine for receptor sites on effector cells.
Phenylpropanolamine stimulates both A and B-adrenergic receptors, similar to ephedrine. Part of its peripheral action is indirect and is due to the displacement of norepinephrine from storage sites, but it also has direct effect on the adrenergic receptors. Codeine is an opiate analgesic and antitussive. Codeine calms the cough control center.