Rhodiola rosea, commonly known as rhodiola, is an adaptogenic herb traditionally used in Eurasian herbal medicine for enhancing stamina and resilience to stress. More recently, its potential effects on the serotonin system and mood regulation have been investigated in scientific studies. Preclinical research indicates that rhodiola extracts may influence neurotransmitter levels in the brain, including serotonin, dopamine, and norepinephrine. Some animal studies have demonstrated that rhodiola can increase serotonin levels in the cortex and hypothalamus, possibly by inhibiting monoamine oxidase (MAO) and facilitating neurotransmitter transport.

Human clinical trials, while limited in size and number, suggest that rhodiola supplementation may help alleviate symptoms of mild to moderate depression and fatigue, potentially through modulation of the serotonin system. For instance, a few randomized controlled trials have shown that rhodiola extract reduces depressive symptoms and improves subjective well-being, with some authors proposing serotonergic mechanisms as a partial explanation. However, direct evidence linking rhodiola's benefits in humans specifically to serotonin modulation is still limited, and most clinical studies use broad measures of mood and stress rather than direct biochemical markers.

In summary, while there is a plausible scientific rationale and limited clinical evidence for rhodiola's support of the serotonin system, more robust and targeted research is needed to confirm these effects and to clarify the underlying mechanisms.