Xanthines

Methylated purine alkaloids that block adenosine receptors: they remove a brake on arousal rather than pressing an accelerator, which is why the stimulation differs in character from a transporter drug's and why the sleep pressure being suppressed is still waiting when the drug wears off. At higher exposures they also inhibit phosphodiesterase, which is where the cardiac and bronchodilator effects come from. Filed under Stimulants by effect, though the subject's dopaminergic and noradrenergic line does not describe them.

Caffeine's own metabolites — paraxanthine, theobromine and theophylline — are active drugs in their own right, and the CYP1A2 enzyme producing them varies between people, is induced by smoking and inhibited by some medicines, so the same amount is not the same exposure in two bodies. Theophylline has a narrow margin with arrhythmia and seizures at the top of it. Dependence and a withdrawal headache are ordinary features of the class.