What this means in practice
Do not add St John's Wort to an SSRI, and do not use it to self-manage coming off one, because tapering involves a period on both. Anyone doing it anyway should know the warning signs: agitation, shivering, tremor, sweating and a racing heart within hours of a dose change.
What people say
A natural antidepressant that can be added on top of a prescription one, or used to taper off it, because a herb and a drug are different sorts of thing and do not compete.
What the evidence supports
Serotonin syndrome or lethargy when St John's Wort is given with serotonin reuptake inhibitors is listed among the four well-documented interaction groups in a pharmacology review of clinical observations, with hyperforin's serotonin reuptake inhibition named as the reason (Borrelli 2009). The UK medicines regulator identified possible pharmacodynamic interactions with SSRIs and with triptans and judged the risk significant enough to inform prescribers and patients (Henderson 2002). Both sources rest on spontaneous case reports, and the regulator's own word is possible rather than established. No controlled study has tested the combination.
How it is supposed to work
Hyperforin inhibits the reuptake of serotonin, which is also what an SSRI does. Taken together the two push serotonin signalling in the same direction, and at the extreme that produces serotonin syndrome, a state of agitation, tremor, fever and rapid heart rate.
Which way it runs
Two-directional and additive, which makes it different in kind from every other interaction on this list. The others are St John's Wort removing a drug; this one is two serotonergic effects stacking. The same reasoning extends to triptans, the migraine drugs, which the UK regulator flagged in the same review.
What is still missing
Herb and drug are different sorts of thing commercially and identical at the receptor. The whole reason St John's Wort works at all is that one of its constituents does what an SSRI does, which is exactly why adding it to one is not a neutral act.
Sources
- Herb-drug interactions with St John's wort (Hypericum perforatum): an update on clinical observationsBorrelli F, Izzo AA, 2009
- St John's wort (Hypericum perforatum): drug interactions and clinical outcomesHenderson L, Yue QY, Bergquist C, Gerden B, Arlett P, 2002