Can I Take St. John's Wort with Topical Minoxidil?

What the topical minoxidil Drug Facts say

The 2% topical solution is sold over the counter; its prescription-to-OTC switch was approved on February 9, 1996 [6]. The men's 5% solution Drug Facts direct 1 mL twice a day onto the top of the scalp only, for men only, and state that the product is not intended for frontal baldness or a receding hairline [1]. They say not to use it when the scalp is red, inflamed, infected, irritated or painful, or when other medicines are being used on the scalp [1]. They also carry a heart-disease warning [1]. The stop-use list includes chest pain, rapid heartbeat, faintness or dizziness, sudden unexplained weight gain, and swelling of the hands or feet [1].

Women's 5% foam is half a capful once daily, with results at about 3 months and up to 6 months for some women [2]. Its Drug Facts carry the same heart-disease warning and the same stop-use signs [2]. The warning about other medicines refers to medicines used on the scalp, and neither label names an oral herbal supplement [1][2].

What St. John's wort does to drug-handling enzymes

St. John's wort activates the pregnane X receptor, which induces CYP3A4 and other cytochrome P450 enzymes and the P-glycoprotein transporter [4]. The size of that effect tracks hyperforin content, not hypericin [4][5]. In one study of 42 healthy men given six different preparations for 14 days, midazolam AUC fell 79.4% with an extract supplying 41 mg of hyperforin a day, 47.9% with a powder supplying 12 mg a day, and 21.1% with a powder almost free of hyperforin at 0.13 mg a day; the correlation was with hyperforin dose (r = -0.765) and not with hypericin [5].

The amount taken also mattered. In 11 healthy volunteers, 14 days of St. John's wort raised oral midazolam clearance 1.96-fold at 300 mg a day, 3.86-fold at 900 mg a day and 5.62-fold at 1,800 mg a day [7]. A separate low-hyperforin powder cut midazolam AUC by 11.3%, which the investigators judged not clinically relevant [8].

These studies show how St. John's wort changes the handling of some drugs, measured with midazolam. They do not show what happens with topical minoxidil, so whether the herb changes hair outcomes or scalp effects is not known.

Oral minoxidil is a separate product

Oral minoxidil tablets have their own FDA approval listing and are not the same product as the 5% topical solution or foam [10]. The tablets are a prescription antihypertensive with a boxed warning, approved only for severe hypertension, and no oral minoxidil product is FDA-approved for hair loss [10]. The topical Drug Facts are written for the topical product and do not cover an oral tablet [1][2].