Lamotrigine does not cause meaningful weight gain. In clinical trials lasting up to a year, people taking lamotrigine averaged a 1.2 kg (about 2.6 pound) loss, which was statistically comparable to placebo. This makes it one of the most weight-neutral options among mood stabilizers and anticonvulsants, and a major reason prescribers favor it for patients concerned about their weight.
What Clinical Trials Show
In two 18-month controlled studies of people with bipolar I disorder, those on lamotrigine lost an average of 1.2 kg at the one-year mark, while people on placebo gained 0.2 kg and people on lithium gained 2.2 kg. The difference between lamotrigine and placebo was not statistically significant, meaning lamotrigine’s effect on weight was essentially indistinguishable from taking a sugar pill.
The rate of clinically significant weight change (defined as gaining or losing 7% or more of body weight) was also comparable between lamotrigine and placebo. In a large open-label study of 1,175 patients, mean weight barely budged after 12 weeks of treatment: from 188.1 pounds at baseline to 188.9 pounds. BMI stayed stable whether patients took lamotrigine alone or alongside other medications like lithium, valproate, antipsychotics, or antidepressants.
How It Compares to Other Medications
Lamotrigine stands out because most other mood stabilizers and many psychiatric medications do cause noticeable weight gain. A large drug safety analysis used lamotrigine as the reference point and found that several common alternatives carried significantly higher rates of severe weight gain:
- Olanzapine: 11.5 times more likely to cause severe weight gain than lamotrigine
- Quetiapine: 3.4 times more likely
- Valproate: 2.4 times more likely
- Lithium: about 2 times more likely, though this didn’t reach statistical significance
Pharmacology reviews consistently classify lamotrigine as “neutral or weight loss” on the spectrum of mood stabilizers. Lithium gained an average of 3.4 kg more than lamotrigine over one year in head-to-head comparison, a difference that was statistically significant.
Why It’s Weight-Neutral
Many psychiatric medications cause weight gain because they affect appetite-regulating pathways, blood sugar control, or fat storage. Lamotrigine works differently. It primarily reduces excess electrical signaling in the brain without strongly interacting with the receptors most linked to appetite and metabolic disruption. A network analysis of metabolic effects across many psychiatric drugs found that lamotrigine did not significantly raise blood sugar, insulin levels, triglycerides, or cholesterol the way medications like olanzapine and risperidone did.
There is one metabolic note worth knowing: that same analysis found lamotrigine may slightly lower HDL (“good”) cholesterol, though the clinical significance of this finding is unclear and it’s a far milder metabolic concern than the broad disruption seen with antipsychotics.
Weight Effects in Children
A prospective study followed 103 children and adolescents on lamotrigine for an average of nearly 19 months. Their weight, height, and BMI scores were essentially identical at the start and end of treatment. Growth proceeded normally regardless of the child’s age, sex, or how long they stayed on the medication. This is particularly relevant because some anticonvulsants used in pediatric epilepsy are known to affect growth trajectories.
Could You Still Gain Weight on Lamotrigine?
The FDA prescribing information does list weight gain as a reported adverse reaction, occurring in more than 1% but fewer than 5% of patients. Individual responses to any medication vary. Some people gain weight while taking lamotrigine for reasons unrelated to the drug itself: changes in mood can affect appetite and activity levels, and other medications taken alongside lamotrigine may independently promote weight gain.
That said, if you’ve recently started lamotrigine and noticed significant weight changes, the medication itself is an unlikely culprit. Long-term treatment has even been associated with modest weight loss in some obese patients with bipolar disorder, though the effect isn’t large enough or consistent enough to consider it a weight-loss medication.
For people switching to lamotrigine from a medication known to cause weight gain, like valproate or olanzapine, weight loss after the switch is common. This isn’t lamotrigine actively burning calories; it’s the removal of the previous drug’s metabolic effects.