I think you are thinking of Zyban. I have never tried it but http://www.netdoctor.co.uk/medicines/100004174.html has all the info.

Chantix anti-smoking pill. Side Affects: The most common adverse event associated with CHANTIX treatment is nausea. For patients treated to the maximum recommended dose of 1 mg BID following initial dosage titration, the incidence of nausea was 30% compared with 10% in patients taking a comparable placebo regimen. In patients taking CHANTIX 0.5 mg BID following initial titration, the incidence was 16% compared with 11% for placebo. Nausea was generally described as mild or moderate and often transient; however, for some subjects, it was persistent throughout the treatment period. Table 3 shows the adverse events for CHANTIX and placebo in the 12 week fixed dose studies with titration in the first week (Studies 2 (titrated arm only), 4, and 5). MedDRA High Level Group Terms (HLGT) reported in ≥ 5% of patients in the CHANTIX 1 mg BID dose group, and more commonly than in the placebo group, are listed, along with subordinate Preferred Terms (PT) reported in ≥ 1% of CHANTIX patients (and at least 0.5% more frequent than placebo). Closely related Preferred Terms such as 'Insomnia', 'Initial insomnia', 'Middle insomnia', 'Early morning awakening' were grouped, but individual patients reporting two or more grouped events are only counted once. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Glaxo Smith Kline’s Zyban. Side Affects: dry mouth insomnia change in appetite agitation headaches Rarely, bupropion can cause seizures, which is why people who have an existing seizure disorder should not take this drug. It is important to carefully follow your doctor's recommendations about dosage due to this risk. Additionally, during clinical studies, the most common side effects which caused people to discontinue use of bupropion chloride were shakiness (jitters) and skin rash.