Antabuse: C10H20N2S4 or bis(diethylthiocarbamyl) disulfide

This compound inhibits the enzyme aldehyde dehydrogenase, which means you can't fully metabolise alcohol, causing a build up of aldehydes. This results in severe nausea and vomiting; think the worst hangover you've ever had! No wonder it makes a good aversion treatment for alcoholism.
Many sulphur containing compounds smell, and by-products of this produced in the body can give the side effect of bad breath.

CH3CH2            CH2CH3            
     \  S     S  /
     /           \
CH3CH2            CH2CH3  

This the story of how this chemical came to be used as a treatment for alcoholism is interesting, it begins soon after WWII in a Danish company, Medicinalco. At that time it was company policy for employees to test new products on themselves, so when Dr Jens Hald and the head of medical research, Dr Erik Jacobsen started taking disulfiram nobody was very much surprised.
They had already established that it killed intesinal parasites in rabbits, and was effective against scabies. In humans however, it seemed to occasionally give rise to a nasty case of vomitting, after lunch! Neither of the Doctors could pin down the exact cause, until Hald had a cognac with a friend. He immediately showed the troublesome side-effects, whilst his friend was fine. The penny dropped.... further conversations with Jacobsen, and then tests on co-workers confirmed that it was the combination of alcohol and disulfiram that caused the sickness! This made it pretty much useless for human parasites, and the project was shelved.

A few years passed, and Jacobsen had started to use the story as an amusing anecdote, which in 1947 the Copenhagen newspaper Berlingske Tidende reported. It was the alcoholics themselves that approached Jacobsen for disulfiram, and after suitable clinical trails it's usage was approved help people break their dependence on alcohol.

'The Consumer's Good Chemical Guide, John Emsley. W.H. Freeman ISBN 0-7167-3034-0 (pbk) pp68-69