St John's wort for PMDD: does it work?

St John's wort hasn't been tested in a trial of diagnosed PMDD. The only placebo-controlled trial of Hypericum perforatum for premenstrual symptoms recruited women with PMS, not PMDD, and it was small.

That trial randomized 36 women with mild PMS to 900 mg a day of standardized Hypericum perforatum (0.18% hypericin, 3.38% hyperforin) or placebo, in a two-cycle crossover design run at the University of Leeds (2010, CNS Drugs) [1].

PMDD is a specific diagnosis: at least five symptoms, one of them a mood symptom, confirmed by daily ratings over two cycles. A trial in PMS doesn't tell us how a treatment performs in PMDD, since the two conditions differ in severity and in how they're diagnosed.

No study has filled that gap since. For what's been tested in PMDD specifically, see PMDD treatment: what actually helps.

The evidence splits into two questions: what's been tested, and whether it worked.

For St John's wort, there's exactly one controlled trial, and it was in PMS. For evening primrose oil, often mentioned alongside it, the picture is messier:

• A 1996 pooled review called the evidence 'of little value' for PMS [2]
• A 1990 crossover trial concluded the improvement women felt was placebo effect alone [3]
• A small subgroup trial of 12 women with PMS found no difference between evening primrose oil (Efamol, combined with the vitamin supplement Efavit) and placebo [4]
• One later trial of 120 students found a bigger drop in symptom severity with evening primrose oil than with placebo [5]

No published trial has tested St John's wort and evening primrose oil taken together, for PMS or for PMDD.

There's no cycle-timed version of St John's wort, the way some SSRIs are prescribed only in the two weeks before a period. The one PMS trial that exists used a daily dose, taken continuously, for two full cycles before anyone switched treatment arms.

• Dose tested: 900 mg a day of standardized Hypericum perforatum (0.18% hypericin, 3.38% hyperforin) [1]
• Schedule: every day of the cycle, not just the premenstrual week [1]
• Duration before comparing results: two menstrual cycles [1]

Reviews of evening primrose oil for women's health conditions generally note that any benefit tends to need 4 to 6 months of steady use rather than a quick fix before a period [6].

If you try an herbal option and want to know whether it's doing anything, give it at least two or three full cycles. Track your days with the Daily Record of Severity of Problems (DRSP) chart before deciding.

St John's wort tends to be mild on its own. Reported side effects include stomach upset, tiredness and increased sensitivity to sunlight.

The bigger concern is what it does to other medications. St John's wort speeds up how your liver breaks down many drugs, which can lower blood levels of hormonal contraceptives, some antidepressants and other medicines. Combined with an SSRI, it can also raise the risk of serotonin syndrome, a rare but serious reaction.

None of the trials covered here, for St John's wort or evening primrose oil, reported side-effect data detailed enough to quote numbers from. That gap is itself part of the problem: small, short trials don't reliably catch rarer or slower-building side effects.

Because St John's wort interacts with so many common medications, tell a doctor or pharmacist before starting it.

Be especially careful if any of these apply:

• You're on a hormonal contraceptive (St John's wort can make it less effective)
• You take an SSRI, SNRI, or another serotonergic medication
• You're on blood thinners, anti-seizure medication, or immunosuppressants
• You're pregnant or breastfeeding, where safety hasn't been established

A PMDD specialist can weigh a herbal option against treatments with stronger trial evidence, given your own history and medications.

Trying St John's wort doesn't rule out everything else. Guidelines for PMDD point to SSRIs (which some people take only in the luteal phase), certain hormonal contraceptives, and CBT, with a doctor deciding what fits your history. PMDD treatment: what actually helps covers those options.

Tracking matters more than which supplement you pick. Daily ratings are how PMDD gets diagnosed in the first place, and they're how you'll know if anything, herbal or prescribed, is changing your week at all. The DRSP chart gives a simple daily format for that.

If you want to see how St John's wort does for milder PMS specifically, St John's wort for PMS: does it work? covers that side of it, and PMS and PMDD research, summarised rounds up more of the evidence base.

St John's wort and evening primrose oil, even if they help, work slowly over cycles. They won't do anything for the moment you're in right now, crying at your desk or replaying a fight at midnight.

For that, something immediate works better: step away for a few minutes, breathe out for a count of six a few times in a row, and write down what's happening before you respond to anyone.