NutriStack
Finasteride and St. John's Wort, a conflict.
St. John's Wort can lower finasteride exposure. In healthy men, 14 days of St. John's Wort pretreatment significantly reduced finasteride Cmax, AUC, and half-life, and a PubMed-indexed clinical correspondence describes a BPH patient with a PSA rise after starting 900 mg/day St. John's Wort while controlled on finasteride. The concern is reduced finasteride effect, with possible worsening BPH control or less reliable PSA suppression.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
Same shape as the other 1,729 pairs in the public database.
From the interaction database
What the row says.
Every entry follows the same shape: what is happening, the mechanism, the recommendation, and the primary literature.
At a glance
- Substances
- Finasteride and St. John's Wort
- Pair type
- Conflict
- Evidence (highest tier)
- Moderate
- Source citations
- 2 sources
- Stack Score effect
- −10 to your Stack Score (per scored conflict row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Conflict · Moderate evidence
Conflict
What is happening. St. John's Wort can lower finasteride exposure. In healthy men, 14 days of St. John's Wort pretreatment significantly reduced finasteride Cmax, AUC, and half-life, and a PubMed-indexed clinical correspondence describes a BPH patient with a PSA rise after starting 900 mg/day St. John's Wort while controlled on finasteride. The concern is reduced finasteride effect, with possible worsening BPH control or less reliable PSA suppression.
Mechanism. Hyperforin-containing St. John's Wort induces CYP3A4 and P-glycoprotein through pregnane X receptor activation. Finasteride is metabolized by CYP3A pathways, and human pharmacokinetic data show reduced finasteride exposure after St. John's Wort induction.
Recommendation. Avoid St. John's Wort while taking finasteride unless the prescriber specifically approves it. If St. John's Wort has already been started or stopped, tell the clinician interpreting PSA or urinary symptom changes because finasteride exposure and PSA suppression may change over several weeks.
Sources (2)
- Lundahl A, Hedeland M, Bondesson U, Knutson L, Lennernas H. The effect of St. John's wort on the pharmacokinetics, metabolism and biliary excretion of finasteride and its metabolites in healthy men. Eur J Pharm Sci. 2009;36(4-5):433-443. PMID 19073252
- Lochner S, Kirch W. [Does St. John's wort interact with finasteride?]. Dtsch Med Wochenschr. 2011;136(34-35):1746. PMID 21877309
Stack Score
How this pair moves the number.
Effect on the composite score
If both Finasteride and St. John's Wort are in the same stack, this pair applies −10 to your Stack Score (per scored conflict row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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