Lorcaserin should not be used in obese patients on SSRIs/SNRIs due to risk of serotonin syndrome.

• A. Topiramate
• B. Lorcaserin
• C. Phentermine
• D. Orlistat

Answer: (B)

When a patient is on SSRIs or SNRIs, serotonin levels are already higher because these drugs prevent serotonin reuptake. Adding another drug that boosts serotonin signaling can push the system into serotonin syndrome, a potentially serious condition characterized by changes in mental status, autonomic instability, and neuromuscular symptoms like tremor or myoclonus.

Lorcaserin works by activating the serotonin 2C receptors in the brain, which increases serotonergic activity. In combination with SSRIs/SNRIs, this dual boosting of serotonin signaling raises the risk of serotonin syndrome, making it the drug to avoid in this situation.

The other medications listed act through different mechanisms and do not significantly add to serotonin signaling in a way that creates this specific interaction risk with SSRIs/SNRIs. Topiramate is a weight-loss agent with anticonvulsant/neuromodulatory effects, phentermine is a sympathomimetic that affects appetite via norepinephrine pathways, and orlistat reduces fat absorption peripherally. None carry the same concern for serotonin syndrome when used with serotonergic antidepressants.