Which medication is least likely to cause weight gain?

Lamotrigine is least likely to cause weight gain among the medications listed. It is primarily used as an anticonvulsant and mood stabilizer, particularly in the treatment of bipolar disorder. Unlike some antipsychotics, which tend to have a higher propensity for inducing metabolic side effects, including weight gain, lamotrigine is generally considered weight-neutral in most patients.

Clozapine and olanzapine, both atypical antipsychotics, are well-documented in their association with weight gain due to their effects on appetite and metabolism. Lithium, although also not typically associated with significant weight gain, can cause weight changes in some individuals due to factors such as fluid retention or increased appetite. In contrast, lamotrigine's mechanism of action and its side effect profile do not include significant weight gain, making it the least likely to cause this issue among the medications mentioned.