Zyban raises the potential for seizures (although the risk is very low). Alcohol use lowers the 'seizure threshold', as does suddenly stopping drinking. Drinking while taking Zyban can make you more dizzy, as well. Let your doctor know if you plan to consume alcohol while taking this medication, and get his/her advice.
Source: Zyban (buproprion hydrochloride) Sustained Release Tablets Prescribing Information. GlaxoSmithKline, Research Triangle Park, NC. 3/2003