Can I Take Magnesium With Finasteride?

Finasteride is a 5-alpha-reductase inhibitor sold as Propecia (1 mg, for androgenic alopecia) and Proscar (5 mg, for benign prostatic hyperplasia). Magnesium is a mineral supplement available in several forms, most commonly citrate, glycinate, and oxide. This page covers whether taking a magnesium supplement alongside either finasteride dose raises any pharmacological concern.

The direct answer

No randomized trial, pharmacokinetic study, or FDA-reviewed drug interaction listing shows that magnesium supplementation changes finasteride blood levels, or that finasteride changes magnesium absorption or excretion. The FDA label for finasteride does not list magnesium or general mineral supplements among its drug interactions. Finasteride is a steroid-like molecule cleared mainly through hepatic CYP3A4 metabolism, not through a pathway magnesium is known to affect at typical supplemental doses. This is an absence-of-evidence situation with a plausible mechanistic explanation, not an unstudied gap that should raise suspicion on its own.

That does not mean the two substances are biologically unrelated. Magnesium participates in insulin signaling and in the availability of free testosterone, both of which sit near the hormonal territory finasteride works in, even though the enzyme finasteride blocks (5-alpha-reductase, which converts testosterone to DHT) is not a step magnesium is known to regulate directly.

What "no direct interaction" does and does not cover

"No direct interaction" means magnesium supplementation is not expected to raise or lower finasteride's plasma concentration, and finasteride is not expected to change how much magnesium you absorb or excrete. It does not mean:

• that every person on finasteride can take any dose of magnesium without individual consideration (kidney function and other medications still matter)
• that magnesium has a proven effect on hair loss or BPH outcomes when added to finasteride (no trial has tested that combination for those endpoints)
• that symptoms occurring while on finasteride can be assumed unrelated to magnesium status, since deficiency symptoms and reported finasteride side effects overlap

How finasteride works, briefly

Finasteride competitively inhibits type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). At 1 mg/day, the FDA label and the Phase III trial data behind it describe substantial reductions in scalp and serum DHT within the first day of dosing. At 5 mg/day for BPH, the PLESS trial (a large randomized, double-blind study, N≈3,040) demonstrated sustained DHT suppression and reduced prostate volume over four years (PLESS trial, NEJM 1998). Finasteride is roughly 90% protein-bound in plasma, mainly to albumin, a binding site magnesium does not compete for at supplemental doses.

Where magnesium is biologically adjacent, without being a direct interaction

Free testosterone and insulin sensitivity. A cross-sectional study of male athletes and sedentary controls found that serum magnesium correlated positively with free and total testosterone, according to a small observational study. This is an observational association from a small study population, not a controlled trial showing that supplementing magnesium raises testosterone in men who are not deficient, and it should be read that way. Separately, a systematic review and meta-analysis of randomized trials has reportedly found that magnesium supplementation improved fasting glucose and insulin resistance markers in people who had insulin resistance at baseline, with effect sizes varying across the included trials. Insulin resistance independently raises sex-hormone-binding globulin (SHBG), which can lower free testosterone, so correcting a real magnesium deficiency in someone with insulin resistance may support the hormonal environment finasteride users often care about. Finasteride itself lowers DHT, not total or free testosterone, so this pathway does not offset finasteride's mechanism; it is a separate, additive consideration.

No effect on DHT itself. There is no human trial evidence that magnesium supplementation raises or lowers serum DHT. Statements to that effect would be extrapolating from mineral cofactor research in cell or animal models, which does not establish a clinical effect in people taking finasteride.

Why this pair is pharmacokinetically unlikely to interact

Finasteride's oral bioavailability is roughly 63-65% and is not meaningfully affected by food. Unlike tetracyclines, fluoroquinolones, or bisphosphonates, finasteride does not have the adjacent carbonyl and amine chelation sites that divalent cations like magnesium bind to, so magnesium is not expected to reduce its absorption (NIH ODS: Magnesium describes this chelation mechanism for other drug classes). Finasteride is metabolized hepatically via CYP3A4 to inactive metabolites and excreted mostly in feces with a smaller urinary fraction; magnesium is cleared renally through glomerular filtration and tubular reabsorption regulated by parathyroid hormone and aldosterone. These are independent systems with no known point of competition at supplemental magnesium doses.

Because there is no absorption interference, dose separation is not pharmacologically required for this pair. Taking both at the same time of day is acceptable; taking them at different times is also fine and carries no known advantage specific to this combination.

When the combination deserves a closer look

The concern in practice is rarely finasteride and magnesium directly. It is what else is in the regimen, and what condition the person has.

Diuretics and proton pump inhibitors (PPIs). Thiazide and loop diuretics cause urinary magnesium wasting, and long-term PPI use is associated with lower serum magnesium in a subset of users, with risk compounded by concurrent diuretic use. Men prescribed finasteride 5 mg for BPH are often older and more likely to be on diuretics or PPIs for unrelated conditions, which is the actual reason this topic comes up in clinical conversation, not a property of finasteride itself.

Chronic kidney disease. The kidneys tightly regulate magnesium excretion. In more advanced CKD, supplemental magnesium can accumulate rather than being cleared normally, raising the risk of hypermagnesemia. Finasteride does not require dose adjustment in renal impairment per its label, but magnesium supplementation in someone with significant CKD should be reviewed by their kidney specialist rather than dosed by general recommendation.

Polypharmacy in older adults. No specific citation in the geriatric prescribing literature reviewed for this page singles out finasteride or magnesium as high-risk in combination. The more relevant point is additive: an older man on finasteride 5 mg, a PPI, and a thiazide diuretic has three independent, unrelated reasons his magnesium could run low, and periodic serum magnesium checmonitoring is a reasonable, low-cost step in that setting.

Symptom overlap: why it's worth checking magnesium status

Fatigue, reduced libido, mood changes, muscle cramps, and sleep disturbance are reported both as finasteride side effects (including in the context of post-finasteride syndrome, a recognized but mechanistically debated clinical entity) and as symptoms of magnesium deficiency. A serum magnesium test is a low-cost, low-risk way to rule deficiency in or out before assuming a symptom is caused by finasteride alone, particularly in someone with a diuretic or PPI on their medication list.

Evidence-status interaction assessment

Magnesium form matters for whether you reach a therapeutic level at all

If magnesium is being added for a real dietary gap rather than out of general habit, the form matters because absorption varies widely.

Magnesium glycinate is absorbed via amino acid transport and tends to cause less osmotic diarrhea at higher doses than oxide, which is why it is often recommended when GI tolerance is a concern.

Magnesium citrate is moderately well absorbed and widely available. A small randomized trial reportedly found citrate produced a greater increase in plasma magnesium than magnesium oxide over 60 days; as a small trial, this should be read as supportive rather than definitive.

Magnesium oxide has the highest elemental magnesium percentage by weight but poor solubility, making it a weak choice specifically for correcting a documented deficiency even though it is the most common form on pharmacy shelves.

The NIH Office of Dietary Supplements sets a tolerable upper intake level of 350 mg elemental magnesium per day from supplements for adults; this ceiling does not apply to dietary magnesium, which has no established upper limit (NIH ODS: Magnesium).

National dietary survey data collected by the CDC (NHANES) is the standard source for how much magnesium US adults typically get from food, though this article does not cite a specific percentage figure from that dataset because the general NHANES portal referenced here does not itself report one (CDC NHANES). Readers who want a precise current shortfall estimate should look at a specific NHANES cycle analysis rather than take a round number as established fact.

Practical guidance on timing and dosing

Because no absorption interference exists between these two, timing is flexible. Finasteride can be taken with or without food at any consistent time. Magnesium is often taken in the evening because of its mild muscle-relaxant effect on sleep, and splitting a daily dose (for example, morning and evening) can reduce loose stools at higher intakes.

The adult male RDA for magnesium from all sources (diet plus supplements) is 400-420 mg/day. Someone eating a varied diet who wants to close a modest gap typically does not need more than 100-200 mg elemental magnesium daily from a supplement; doses used to treat a documented deficiency are higher and should be guided by lab values and a clinician, not by a general online recommendation. This article does not provide individualized dosing for a specific person's lab results or comorbidities.

Interactions that actually matter, even though they are not this one

• Finasteride with warfarin: a single case report described an elevated INR in a patient on both drugs, but causation was not established. Anyone on warfarin starting finasteride should mention it to the prescriber managing anticoagulation, independent of the magnesium question.
• Magnesium with tetracyclines or fluoroquinolones: magnesium chelates these antibiotics and reduces their absorption; standard practice is to separate dosing by at least two hours (NIH ODS: Magnesium).
• Magnesium with bisphosphonates: the same chelation concern applies; separate by at least two hours.
• Magnesium with calcium channel blockers: high magnesium doses can have additive blood-pressure-lowering and heart-rate effects; this is worth mentioning to a prescriber if magnesium doses are being increased.

Guideline context on correcting deficiencies before hormonal treatment

While the Endocrine Society's 2018 clinical practice guideline on testosterone therapy recommends evaluating correctable metabolic and nutritional factors, it does not specifically address magnesium supplementation (Endocrine Society CPG, JCEM 2018). Applying this principle to finasteride users by considering magnesium status represents a logical extrapolation based on magnesium's known effects on insulin sensitivity and testosterone bioavailability, rather than an evidence-based guideline recommendation, and reflects clinical judgment rather than an established standard of care.

Who should talk to a prescriber before combining them

Most healthy adults taking finasteride 1 mg for hair loss, with normal kidney function and no diuretic or PPI use, can add a standard magnesium supplement without a dedicated physician visit. The following groups should check first:

• Anyone with CKD stage 3b or worse, where supplemental magnesium above roughly 200 mg elemental per day carries a real accumulation risk
• Anyone on a thiazide or loop diuretic long-term, where magnesium loss can be greater than expected and monitoring is reasonable
• Anyone on a PPI for an extended period, where hypomagnesemia risk rises with duration of use
• Anyone with symptoms suggestive of post-finasteride syndrome, where a full workup (including magnesium) should precede attributing symptoms to the drug or to a supplement
• Older adults on finasteride 5 mg plus an alpha-blocker and other cardiovascular medications, given the cumulative comorbidity burden in that group

If new muscle weakness, irregular heartbeat, severe nausea, or confusion develop while taking magnesium, that warrants urgent medical evaluation rather than waiting for a routine follow-up, since these can be signs of significant hyper- or hypomagnesemia.

Summary of the interaction risk profile