Zyban^® (bupropion SR) is a prescription drug used to help people quit smoking. Often combined with therapy or other forms of counseling, the drug blocks the reuptake of certain chemicals in the brain. Zyban comes in tablet form, and is usually taken twice a day. Side effects of Zyban can include insomnia, constipation, and problems concentrating. A few of the factors that can affect the Zyban dosage your healthcare provider prescribes include your age, other medical conditions you may have, and other medications you are currently taking.