Minoxidil and Finasteride: How They Work, Side Effects and Who Should Avoid Them

Minoxidil and finasteride are the two medicines most often discussed for pattern hair loss. The NHS describes them as the main treatments for male pattern baldness, with the caveat that they do not work for everyone and only work while you keep using them. They act in very different ways, carry different risks and suit different people. This guide sets out what each one does, where newer options such as low-dose oral minoxidil and dutasteride fit, and who should avoid them.

How minoxidil works

Minoxidil is a prodrug, meaning it has to be converted before it becomes active. A review in Postepy Dermatologii i Alergologii explains that an enzyme in the hair follicle, called sulfotransferase (SULT1A1), turns minoxidil into minoxidil sulfate. Levels of this enzyme vary widely between people, and researchers think this helps explain why some people respond well and others barely respond at all.

Topical minoxidil

Topical minoxidil comes as 2% and 5% products. The same review notes that men typically use 5% once or twice daily, and that in women 2% twice daily has been described as comparable to 5% once daily. The American Academy of Dermatology (AAD) states that both strengths are FDA-approved for women and that results take 6 to 12 months of regular use.

Reported side effects of the topical form include:

• Irritant or allergic contact dermatitis on the scalp
• Unwanted hair growth elsewhere (hypertrichosis)
• A temporary increase in shedding, which the review places around 6 to 8 weeks after starting
• Increased hair loss after stopping

The review says topical minoxidil is not recommended during pregnancy or breastfeeding, and the AAD gives the same advice.

Low-dose oral minoxidil

Oral minoxidil was developed as a blood pressure drug. A 2025 narrative review in the Journal of Clinical Medicine states that it is FDA-approved only for severe, treatment-resistant high blood pressure, so using it at low doses for hair loss is off-label. The review summarises the doses reported in the literature.

The same review reports these adverse events:

• Unwanted hair growth: about 15% in the largest study, dose-dependent and more often noticed by women
• Fluid retention or leg swelling: 1.3% to 10% across studies, usually appearing in the first one to three months
• Headache: 0.4% to 9%
• Fast heart rate: about 0.9% to 4%, usually mild and early
• Dizziness: about 1% to 1.7%, usually in the first week

According to the review, most of these resolved after a dose reduction, without stopping treatment. The authors list pheochromocytoma (a rare adrenal tumour) and allergy to the drug as contraindications. They do not recommend low-dose oral minoxidil after a recent heart attack, with heart failure involving reduced pumping function, with severe heart valve disease or with advanced kidney disease without specialist input. They advise caution in people with high blood pressure, irregular heart rhythm, low blood pressure on standing, existing swelling or pericardial disease. Routine blood tests are not required, but the review suggests checking heart history and blood pressure before starting.

How finasteride works

DermNet describes finasteride as a type II 5-alpha reductase inhibitor. It blocks the conversion of testosterone to dihydrotestosterone (DHT), the hormone that drives male pattern hair loss, lowering DHT by up to 70% in the blood and 60% in the scalp. The usual dose for hair loss is one 1 mg tablet a day.

DermNet notes that three months or more of daily use may be needed before any effect shows, and that treatment probably has to continue indefinitely. When it stops, DHT rises again and hair loss is likely to resume.

Side effects

In trials summarised by DermNet, sexual side effects were uncommon and generally resolved after stopping:

Breast enlargement has also been reported. After the drug reached the market, there were reports of depression and, more rarely, anxiety, panic attacks and suicidal thoughts. DermNet also points out that finasteride lowers PSA, a blood test used in prostate cancer screening, by about 50% on average, which matters for anyone having that test.

Oral versus topical finasteride

The FDA has approved only oral finasteride tablets. In an April 2025 alert, the agency stated that no topical finasteride product is FDA-approved and that compounded topical versions have not been evaluated for safety, effectiveness or quality. It received 32 adverse event reports for these products between 2019 and 2024, and many reported effects continued after people stopped using them.

Dutasteride

Dutasteride blocks both type I and type II 5-alpha reductase. The SULT1A1 review reports dose-dependent results at 0.5 mg a day and above, and describes it as more effective than finasteride with comparable side effects. The AAD lists it among tablets sometimes prescribed for female pattern hair loss, while noting that it is not FDA-approved for that use. The review advises women of reproductive age to use contraception during treatment and for six months afterwards.

Who should not use them

• Pregnancy: DermNet states that women who are or may be pregnant must not use finasteride because it can cause genital abnormalities in a male fetus. The FDA adds that women should not handle crushed or broken tablets, and that topical products carry a higher risk of transfer to other people through skin contact.
• Women in general: The NHS says women should not use finasteride. The AAD notes that some dermatologists prescribe these tablets to women off-label, with birth control for anyone who could become pregnant.
• Children: DermNet states that finasteride is not indicated for children.
• Heart and kidney conditions: see the cautions for oral minoxidil above.
• Breastfeeding: the AAD advises avoiding topical minoxidil during pregnancy and breastfeeding.

Can tretinoin boost topical minoxidil?

Because minoxidil depends on follicle sulfotransferase, researchers have looked for ways to raise that enzyme. The SULT1A1 review describes a small study by Sharma and colleagues in which topical tretinoin changed follicular sulfotransferase expression, and 43% of people predicted to be non-responders to minoxidil became predicted responders after five days of tretinoin. The review treats this as a possible add-on approach and notes that the supporting evidence is still limited. Anyone considering the combination should discuss it with a dermatologist first.

Bottom line

Topical minoxidil is widely available and suits most adults who are not pregnant or breastfeeding. Oral minoxidil, finasteride and dutasteride are prescription decisions that depend on sex, pregnancy plans and heart history. All of them need months of steady use and stop working when they are stopped.