At a glance
- Evidence for St John's wort in PMS is mixed: one open trial found big improvement, but two placebo-controlled trials found only a weak or non-significant effect.
- Studied doses were 600 mg/day or 900 mg/day of standardized extract, taken daily across the whole cycle, not just before a period.
- Common side effects reported were nausea, headache, dizziness, fatigue and dry mouth.
- It speeds up a liver enzyme that processes many drugs, including some antidepressants and chemotherapy medicines, so check with a doctor before combining it with anything else.
- If you try it, give it at least two full cycles and track symptoms daily to see if it's actually helping.
Strength of evidence
Mixed evidence
Trials are small and disagree. An open trial with no placebo group found a large improvement; a placebo-controlled trial found only a non-significant trend, and a second placebo-controlled crossover trial added more data without settling the question. A 2006 review calls the evidence promising but incomplete, so no one can say with confidence that St John's wort works for PMS yet.
Does St John's wort work for PMS?
The evidence is mixed, not settled. St John's wort (Hypericum perforatum) affects serotonin, the same system SSRIs work on, which is why researchers have tested it for PMS ( [1], [4]).
One open trial, which had no placebo group to compare against, found a large drop in symptoms ( [3]). Two later trials added a placebo group: one found only a non-significant trend toward improvement ( [2]), and the other, a double-blind crossover trial, added more controlled data without settling the question ( [1]).
A 2006 review summed it up plainly: St John's wort shows possible benefit for PMS, but more research is needed before anyone can call it reliable ( [4]). This page covers PMS; the picture for PMDD specifically is different and sits on its own page about St John's wort for PMDD.
What does the research say?
Four pieces of evidence shape what we know, and they don't all point the same way.
| Study | Design | What it found |
|---|---|---|
| 2000, BJOG (19 completed) | Open trial, no placebo group | Symptom score fell 49%, but the authors warned placebo effect and natural symptom swings could explain much of that ( [3]) |
| 2004, J Altern Complement Med (125 completed) | Double-blind, placebo-controlled, 600 mg/day | Trend toward improvement in anxiety-related symptoms, not statistically significant ( [2]) |
| 2010, CNS Drugs (n=36) | Double-blind, placebo-controlled crossover, 900 mg/day | Added more controlled data to a still-unsettled picture ( [1]) |
| 2006, J Society for Integrative Oncology | Narrative review | Says St John's wort has possible efficacy for PMS, but calls for more research ( [4]) |
None of these trials are large by modern standards, and none have been repeated at scale. If you're curious how researchers weigh findings like these, Niyora's research page explains how evidence gets graded for PMS and PMDD treatments.
How long does it take to work?
Both placebo-controlled trials followed women for two menstrual cycles on each treatment before comparing results ( [1], [2]). That's a reasonable amount of time to judge whether it's working for you too.
PMS symptoms naturally shift from month to month, so judging a treatment after a single cycle can be misleading. Tracking your symptoms daily for at least two cycles gives you a clearer picture than relying on memory.
A DRSP chart makes that tracking easier: you rate the same symptoms every day, so you can compare one cycle against the next instead of guessing.
What are the side effects?
The side effects reported in reviews of St John's wort are usually mild ( [4]):
- Nausea
- Headache
- Constipation
- Dizziness
- Confusion
- Fatigue
- Dry mouth
St John's wort is broken down mainly by the liver, and it speeds up an enzyme (CYP3A4) that also processes many prescription drugs ( [4]). That interaction matters more than the mild side effects above for most people.
Who should avoid it?
St John's wort changes how your liver clears other drugs, so it can make them less effective or, in some combinations, risky ( [4]). These combinations need extra caution:
- You take an SSRI, another antidepressant, or a triptan for migraines (combining raises the risk of serotonin syndrome)
- You take antiretrovirals, cyclosporine, tacrolimus, antiepileptics, irinotecan, or other chemotherapy drugs (St John's wort can lower how well these work)
- You take any other medication regularly, since the interaction list is long and still growing
If your symptoms feel heavier than typical PMS, for example if they take over the week before your period and ease only once it starts, a PMDD specialist can help you figure out what's actually going on before you try a supplement.
What else helps alongside it?
St John's wort is one option among several, and it's rarely the first thing doctors reach for. Guidelines point to a few approaches with more consistent evidence behind them: SSRIs, which can work even when taken only in the second half of the cycle, certain hormonal contraceptives, and cognitive behavioral therapy.
A doctor is the right person to decide what fits your situation, especially since some of these can be combined safely and others can't. PMS treatment, explained walks through the full range of options side by side.
Regular sleep and exercise also show up across the PMS research, though their effect tends to be smaller than medication. None of this replaces a conversation with a doctor about what's actually going on for you.
What helps in the moment?
Supplements and prescriptions work over weeks, not in the ten minutes when you're already crying or snapping at someone you love. For that, you need something immediate.
- Name what's happening out loud or on paper: "I'm two days from my period and I feel like everything is too much."
- Step away from the conversation or task for 20 minutes if you can. Set a timer so it's not open-ended.
- Write down what actually happened before you respond to anyone, so you're working from facts instead of the worst version of the story.
If you want somewhere to put a fight before you answer, Niyora is a PMS and PMDD support app. Its Moon feature lets you write down what happened, see it from another angle, and draft a reply you can hold for 20 minutes before sending anything.
Questions people ask
Does St John's wort cure PMS?+
No. St John's wort hasn't been shown to cure PMS in any trial. The evidence is mixed: one open trial found a big improvement, but placebo-controlled trials found only a small or non-significant effect. Treat it as one option among several, not a guaranteed fix.
What dose of St John's wort was used in PMS studies?+
PMS trials used standardized extracts, most often 900 mg/day or 600 mg/day, taken daily through the whole cycle rather than just the days before a period. Doses varied by study, and none set an official recommended dose, so a doctor should guide dosing if you try it.
How long before St John's wort helps PMS symptoms?+
St John's wort trials for PMS tracked women for two menstrual cycles on each treatment before comparing results. That's a reasonable amount of time to judge whether it's working for you too, since PMS symptoms naturally vary from month to month.
Can I take St John's wort with an SSRI for PMS?+
Combining St John's wort with an SSRI or another antidepressant raises the risk of serotonin syndrome, a serious reaction. St John's wort also changes how the liver processes many other drugs. See a doctor before combining it with any prescription medication.
Does St John's wort work for PMDD too?+
The evidence for PMDD is thinner than for PMS: one case report describes improvement after switching from an SSRI, but there's no placebo-controlled trial. PMDD and PMS are different diagnoses, and the research is covered separately on the St John's wort for PMDD page.
Sources
- 1.The efficacy of Hypericum perforatum (St John's wort) for the treatment of premenstrual syndrome: a randomized, double-blind, placebo-controlled trial, CNS drugs, 2010
- 2.The significance of "nonsignificance" in randomized controlled studies: a discussion inspired by a double-blinded study on St. John's wort (Hypericum perforatum L.) for premenstrual symptoms, Journal of alternative and complementary medicine (New York, N.Y.), 2004
- 3.St John's Wort and the premenstrual syndrome, BJOG An International Journal of Obstetrics & Gynaecology, 2000
- 4.About the cover: St. John's wort, Journal of the Society for Integrative Oncology, 2006
- 5.St. John's wort (Hypericum perforatum) as a treatment for premenstrual dysphoric disorder: case report, International journal of psychiatry in medicine, 2003