It also is sold without a prescription in products marketed to treat drowsiness. Caffeine was first approved by the FDA for use in a drug product in 1938. Caffeine is a mild, direct stimulant at all levels of the CNS and also stimulates the heart and cardiovascular system. The related xanthine, theophylline, shares these properties and is widely used in the treatment of pulmonary disease. Both caffeine and theophylline are CNS stimulants, with theophylline exerting more dramatic effects than caffeine at higher concentrations.
Caffeine also stimulates the medullary respiratory center and relaxes bronchial smooth muscle.
Caffeine stimulates voluntary muscle and gastric acid secretion, increases renal blood flow, and is a mild diuretic. While the clinical responses to caffeine are well known, the cellular mechanism of action is uncertain. Several theories have been proposed. At high concentrations, caffeine interferes with the uptake and storage of calcium by sarcoplasmic reticulum of striated muscle. While this action would explain the effects of caffeine on cardiac and skeletal muscle, it does not appear to occur at clinically achievable concentrations.