Three formulations of bupropion are available: immediate release (taken three times daily), sustained release (taken twice daily), and extended release (taken once daily). The different versions of the drug contain the same active ingredient but differ in their pharmacokinetics and dosing. There have been reports of inconsistencies in bioequivalence between various branded and generic versions of bupropion. Any changes with this drug in tolerability or clinical efficacy in a patient who had been doing well should prompt an inquiry about whether these changes correspond to a switch to a new formulation.

Immediate-release bupropion is well absorbed from the gastrointestinal (GI) tract. Peak plasma concentrations of bupropion are usually reached within 2 hours of oral administration, and peak levels of the sustained-release version are seen after 3 hours. The mean half-life of the compound is 12 hours, ranging from 8 to 40 hours. Peak levels of extended-release bupropion occur 5 hours after ingestion. This provides a longer time to maximum plasma concentration (t_max) but comparable peak and trough plasma concentrations. The 24-hour exposure occurring after administration of the extended-release version of 300 mg once daily is equivalent to that provided by sustained release of 150 mg twice daily. Clinically, this permits the drug to be taken once a day in the morning. Plasma levels are also reduced in the evening, making it less likely for some patients to experience treatment-related insomnia.

The mechanism of action for the antidepressant effects of bupropion is presumed to involve the inhibition of dopamine and norepinephrine reuptake. Bupropion binds to the dopamine transporter in the brain. The effects of bupropion on smoking cessation may be related to its effects on dopamine reward pathways or to inhibition of nicotinic acetylcholine receptors.