Wellbutrin XL and Wellbutrin SR are different formulations of the antidepressant bupropion. Both Wellbutrin XL and Wellbutrin SR may cause weight gain or weight loss, but weight loss is more common. Not everyone taking these drugs will experience a change in weight.

Weight changes are considered a side effect for both Wellbutrin XL and Wellbutrin SR. Studies of Wellbutrin XL show that 23% of people taking a dose of 150 to 300 mg per day lost 5 pounds or more. In the same studies, 11% of people gained more than 5 pounds.

Studies of Wellbutrin SR show that 14% of people taking a dose of 300 mg per day lost more than 5 pounds. At a dose of 400 mg, 19% of people lost more than 5 pounds. In the same studies, a small percentage of people gained more than 5 pounds: 3% taking 300 mg and 2% taking 400 mg.

Bupropion may also be used to help people stop smoking. A study of sustained-release (SR) bupropion for smoking cessation found that people using this medication were less likely to gain weight after they stopped smoking.

The reason bupropion causes some people to lose weight is not known, but it may have to do with its mechanism of action to treat depression.

Bupropion increases the level of the brain chemical dopamine. Some studies suggest that lower levels of dopamine may be linked to obesity.

The difference between Wellbutrin XL and Wellbutrin SR lies in the dosing schedule. XL is an extended-release medication and only needs to be taken once a day. SR stands for sustained release, and it may be taken twice a day.

Both XL and SR formulations are approved to treat major depressive disorder. Wellbutrin XL has an additional approval for seasonal affective disorder (SAD), which is depression that occurs with the changing of seasons, usually in the fall and winter.