Mesocarb (Sidnocarb, Sydnocarb) is a stimulant drug which was developed in the USSR in the 1970s. It has been shown to act as a dopamine reuptake inhibitor which is slower acting but longer lasting and less neurotoxic than dextroamphetamine.

Mesocarb is still used for a variety of applications; these include counteracting the sedative effects of benzodiazepine drugs, increasing workload capacity and cardiovascular function, treatment of ADHD and hyperactivity in children, as a nootropic, and as a drug to enhance resistance to extremely cold temperatures. It is also listed as having antidepressant and anticonvulsant properties.

Mesocarb is sold in Russia as 5 milligram tablets under the brand name Sydnocarb. Hydroxylated metabolites can be detected in urine for up to 10 days after consumption, reflecting a relatively long half-life.

Mesocarb is almost unknown in the western world and is neither used in medicine or studied scientifically to any great extent outside Russia and other countries in the former Soviet Union. It has however been added to the list of drugs under international control and is illegal in most countries, despite its multiple therapeutic applications and the lack of significant abuse potential seen in clinical practice.