What happens when you take digoxin with St. John's wort?
Digoxin is a cardiac glycoside used to slow the ventricular response in atrial fibrillation and provide modest inotropic support in heart failure. Unlike most drugs, digoxin is not significantly metabolized by cytochrome P450 enzymes. Its absorption, distribution, and renal excretion all depend on a transporter called P-glycoprotein (P-gp), which acts as an efflux pump in the gut, biliary tract, blood-brain barrier, and kidney tubules.
St. John's wort (Hypericum perforatum) is a popular over-the-counter herb used for low mood. Its active constituent hyperforin activates a nuclear receptor called PXR, which dramatically increases the expression of P-glycoprotein and CYP3A4 in the intestine. For digoxin, the relevant pathway is P-gp: more P-gp in the gut wall pumps absorbed digoxin back out into the lumen before it can enter the bloodstream.
A controlled study by Durr and colleagues, and follow-up work by Johne and colleagues, found that 14 days of standardized St. John's wort produced a roughly 25 percent drop in digoxin AUC and a 30-36 percent drop in peak plasma concentration in healthy volunteers. The effect builds over about two weeks and washes out over a similar period after stopping the herb.
Why is this important?
Digoxin has a narrow therapeutic window. Target serum levels for heart failure are typically 0.5-0.9 ng/mL, with toxicity becoming likely above 2.0 ng/mL. A 25-30 percent drop in concentration can push a patient from therapeutic to sub-therapeutic, which means inadequate rate control in atrial fibrillation (with a fast irregular pulse and palpitations returning) or worsening heart failure symptoms.
The reverse situation — stopping St. John's wort while continuing the increased digoxin dose — is also risky. When the herb is withdrawn, P-gp expression normalizes over two weeks. The previously needed higher digoxin dose can then become excessive and produce toxicity, with nausea, visual disturbances, and life-threatening arrhythmias.
This interaction is particularly insidious because patients often do not consider herbal supplements to be drugs and may not mention them when their prescriber reviews their medication list. A patient whose digoxin level seems to be drifting downward without a clear reason should be asked specifically about herbal and over-the-counter supplements.
What should you do?
Do not take St. John's wort while on digoxin. If you are starting digoxin and have been using St. John's wort, stop the herb and wait at least two weeks before the prescriber relies on a steady-state digoxin level for dosing decisions. If you have been taking both and want to continue digoxin, stop St. John's wort and have a digoxin level checked in 1-2 weeks so the prescriber can adjust the dose if it climbs too high.
Choose an evidence-based alternative for low mood if needed: cognitive behavioral therapy, exercise, social support, or a prescription antidepressant chosen for compatibility with your other medications. Discuss the choice with the prescriber managing your heart condition.
Which specific products are affected?
St. John's wort products vary widely in hyperforin content. Higher-hyperforin preparations (typically standardized to 3-5 percent hyperforin) produce the strongest induction. Low-hyperforin formulations cause less interaction but are still considered unsafe with digoxin because hyperforin content varies between batches and product testing has shown labels can be unreliable. Combination herbal sleep, mood, or stress products often contain St. John's wort as one of several ingredients; check labels carefully.
Digoxin products affected include Lanoxin tablets, Digox tablets, generic digoxin tablets, and pediatric digoxin solution. Other P-glycoprotein substrates are also affected by St. John's wort, including the direct oral anticoagulants dabigatran and edoxaban, the immunosuppressants cyclosporine and tacrolimus, and the protease inhibitors used in HIV treatment.
The bottom line
Digoxin and St. John's wort is a high-severity interaction. The herb induces intestinal P-glycoprotein, increasing efflux of digoxin and lowering its bioavailability by roughly 25-30 percent over two weeks of use. This can cause therapeutic failure in atrial fibrillation rate control and heart failure management. Avoid the combination, disclose all supplement use to your prescriber, and allow at least two weeks of wash-out after stopping St. John's wort before assuming digoxin levels reflect a stable steady state.