Which of the following medications could be contraindicated with SSRIs due to the risk of serotonin syndrome?

St. John’s Wort is known to have serotonergic properties, which means it can increase serotonin levels in the brain. When used in combination with selective serotonin reuptake inhibitors (SSRIs), there is a significant risk of serotonin syndrome, a potentially life-threatening condition characterized by symptoms such as agitation, confusion, rapid heart rate, increased blood pressure, dilated pupils, and muscle rigidity. The mechanism by which St. John’s Wort can contribute to this condition involves its ability to induce enzymes that affect the metabolism of SSRIs, leading to higher concentrations of serotonin.

The other choices do not have the same level of concern regarding serotonin syndrome. Ginkgo biloba is often considered safe but can interact with blood thinners. Valerian root is commonly used for its sedative properties but does not notably affect serotonin levels. Chamomile is primarily used for relaxation and anxiety relief and does not significantly influence serotonin levels either. Hence, the primary concern with St. John’s Wort in combination with SSRIs arises from its potential to elevate serotonin levels, making it contraindicated in this context.