Name a drug that can cause serotonin syndrome when combined with SSRIs and explain its mechanism.

• A. Vancomycin; it neutralizes SSRIs.
• B. Penicillin; it enhances SSRIs.
• C. Amlodipine; it inhibits SSRIs.
• D. Linezolid; it has MAOI activity that increases serotonin levels when combined with SSRIs.

Answer: (D)

Serotonin syndrome arises when there is an excess of serotonin in the brain, often from combining drugs that raise serotonin through different mechanisms. SSRIs increase serotonin levels by blocking its reuptake, leaving more serotonin available at nerve synapses. If you add another drug that prevents serotonin from being broken down, the total serotonin signal can spike.

Linezolid stands out because it has monoamine oxidase inhibitor activity. MAO enzymes normally break down serotonin; when MAO is inhibited, serotonin isn’t metabolized as quickly, so its levels rise. Using linezolid together with an SSRI blocks two different steps in the same pathway—reuptake and breakdown—leading to a substantial increase in serotonin signaling and a real risk of serotonin syndrome. This is why linezolid is the classic example of a drug that can trigger the syndrome when paired with SSRIs.

The other drugs don’t carry this risk through MAOI activity. They don’t inhibit serotonin metabolism in the same way, so they don’t create the same synergistic surge with SSRIs.