Along with its needed effects, bupropion may cause some unwanted or undesirable effects. Generally, bupropion is well tolerated and many people will not experience unwanted effects. The frequency and severity of these effects is dependant on many factors including dose, duration of therapy and individual susceptibility. Possible unwanted effects include:

• dry mouth
• trouble sleeping
• runny nose
• dizziness
• nausea
• constipation

Uncommon:

• vomiting
• agitation
• headache
• tremor
• rashes

Rare but serious side effects:  Seek medical help if you experience:

• allergy (e.g. itching, hives, rash, difficulty breathing)
• increased blood pressure
• seizures
• new or worsened emotional or behavioural problems, such as hostility, agitation, thoughts of suicide.
• peeling of the skin, or rash combined with blisters around the mouth, eyes or genitals.

Bupropion modifies the effect of certain chemical messengers in the central nervous system. Its exact mechanism as a smoking cessation aid isn't clear but may involve changes in norepinephrine and/or dopamine pathways. It has been suggested that dopamine plays a role in the reinforcement of addiction and norepinephrine in the withdrawal symptoms that occur when the addicting agent is discontinued. Bupropion reduces the craving for nicotine and the withdrawal symptoms (e.g. irritability, anxiety, depressed mood) that may occur when smoking is stopped.

• Allergic reactions occur rarely with bupropion use. Symptoms include hives, itching, rashes, swelling, and shortness of breath. If you experience these symptoms, consult with your doctor immediately.
• Bupropion does not contain nicotine and therefore is sometimes used with nicotine replacement agents (e.g., patches, gum). In a few people, this combination has been associated with an increase in blood pressure. If you are using both bupropion and nicotine replacement agents your doctor may want to measure your blood pressure more frequently.
• Rarely, people taking bupropion will have a seizure. This chance increases if you have a seizure disorder (e.g. epilepsy), eating disorder (e.g. anorexia), take other medications that may increase your chance of a seizure, have liver problems, have had a serious head injury, have diabetes which is being treated with insulin or other medications. It is best not to drink alcohol at all or to drink very little while taking bupropion. If you drink a lot of alcohol and suddenly stop, you may increase your chance of having a seizure.
• Like all drugs which affect the central nervous system, bupropion can reduce mental alertness and physical coordination particularly when you first begin taking the medication. Avoid activities such as driving a car or operating complex or heavy machinery until you know how the drug affects you. 

Drug Interactions: Bupropion can affect the way your body handles certain other drugs. It is very important that you inform your doctor and pharmacist of all the prescription and over-the-counter medications that you are taking. The following drugs and drug classes may interact with bupropion hydrochloride:

• alcohol
• anticonvulsants 
• antidepressants 
• antipsychotics 
• levodopa 
• orphenadrine 
• prednisone
• theophylline

Use is not recommended in the following situations:

• allergy to bupropion or any of the components of the medication
• seizure disorders 
• eating disorders 
• current use (or within 14 days of stopping) a monoamine oxidase (MAO) inhibitor
• current use of bupropion as an antidepressant

Caution is recommended in the following situations:

• hypertension
• heart disease
• liver disease
• kidney disease

Use in pregnancy: Current information suggests low risk. Consult your doctor or pharmacist if you are or plan to become pregnant.
Use while breast-feeding: Bupropion is passed into breast milk. Consult your doctor or pharmacist before taking bupropion while breast-feeding.
Use in children: The use of bupropion in people younger than 18 years of age has not been adequately studied. Consult your doctor or pharmacist.