St John’s Wort does not raise blood pressure on its own. Clinical testing has shown no effect on blood pressure, heart rate, or cardiovascular regulation in humans taking the supplement by itself. However, the picture gets more complicated when you factor in food interactions, other medications, and the supplement’s powerful effect on how your body processes drugs. In certain combinations, St John’s Wort can contribute to dangerous blood pressure spikes.
What the Clinical Evidence Shows
A controlled study measuring St John’s Wort’s influence on cardiovascular regulation found no changes in systolic or diastolic blood pressure, heart rate, or heart rate variability under any test condition. Taken alone, at standard doses, the supplement appears to be cardiovascularly neutral. If you’re otherwise healthy and not taking other medications, St John’s Wort is unlikely to move your blood pressure readings in either direction.
The Tyramine Risk
St John’s Wort acts as a monoamine oxidase (MAO) inhibitor, meaning it slows down an enzyme your body uses to break down certain chemicals in your brain and bloodstream. One of those chemicals is tyramine, an amino acid found naturally in aged and fermented foods like aged cheese, cured meats, soy sauce, red wine, and draft beer.
Normally, your body breaks down tyramine quickly before it can affect your cardiovascular system. But when an MAO inhibitor is present, tyramine builds up. High tyramine levels trigger a sudden release of stress hormones that constrict blood vessels, and the result can be a hypertensive crisis: a rapid, severe spike in blood pressure that can cause headache, chest pain, and in extreme cases, stroke. At least one documented case involved a young woman who experienced a hypertensive crisis while taking St John’s Wort, with excess tyramine-containing foods identified as a likely contributor.
This doesn’t mean every meal with cheese becomes dangerous. The risk depends on how much tyramine you consume and how strongly the supplement is inhibiting the enzyme. But if you take St John’s Wort regularly, it’s worth being aware that large amounts of tyramine-rich foods carry a real, if uncommon, risk of sharp blood pressure increases.
Serotonin Syndrome and Blood Pressure
Combining St John’s Wort with antidepressant medications, particularly SSRIs and SNRIs, can trigger serotonin syndrome. This is a condition where too much serotonin accumulates in the brain and body. Early symptoms include a fast heart rate, elevated blood pressure, dilated pupils, and heavy sweating. In severe cases, it can progress to dangerously high fever, seizures, and muscle rigidity.
The blood pressure increase in serotonin syndrome isn’t a gradual change you’d notice over weeks. It comes on quickly and is part of a cluster of symptoms that feel unmistakably wrong. This interaction is well-established enough that taking St John’s Wort alongside prescription antidepressants is widely considered unsafe.
How It Can Undermine Blood Pressure Medications
Perhaps the most practically important risk for people with high blood pressure is that St John’s Wort can make their medications less effective. The supplement is a potent activator of liver enzymes (specifically CYP3A4) and a protein called P-glycoprotein, both of which help your body clear drugs from your system. When St John’s Wort ramps up these pathways, certain medications get broken down and eliminated much faster than they should be.
Several common blood pressure drugs are affected:
- Verapamil: A calcium channel blocker used for hypertension. Two weeks of St John’s Wort reduced the amount of verapamil in the bloodstream by 78 to 80%, essentially gutting its effectiveness.
- Nifedipine: Another calcium channel blocker, also showed significantly reduced blood levels when taken with the supplement.
- Talinolol: A beta blocker used for hypertension, which saw a 31% reduction in blood levels and a 93% increase in the rate the body cleared it after just 12 days of St John’s Wort use.
The practical consequence is straightforward. If you take a blood pressure medication that’s working well and then start St John’s Wort, your blood pressure could rise, not because the supplement is actively pushing it up, but because your medication is no longer reaching effective levels in your body. This can happen within one to two weeks and may not be obvious unless you’re checking your blood pressure at home. It can also persist for a period after you stop the supplement, since the enzyme changes don’t reverse instantly.
Who Should Be Cautious
If you have normal blood pressure, don’t take antidepressants, and eat a typical diet, St John’s Wort is unlikely to affect your cardiovascular numbers. The direct evidence is reassuring on that front.
The people who need to pay attention are those already managing high blood pressure with medication, those taking any form of antidepressant, and those who regularly eat large amounts of aged or fermented foods. In each of these situations, the risk isn’t from the supplement alone but from its interaction with something else in your body or your diet. The effects can range from a gradual loss of blood pressure control (if medications are being neutralized) to a sudden, dangerous spike (if tyramine accumulates or serotonin syndrome develops).