Dutasteride vs Finasteride: Differences, Effectiveness and Safety in Male Pattern Hair Loss

Dihydrotestosterone (DHT) is made from testosterone by the enzyme 5-alpha-reductase. In people whose hair follicles are genetically sensitive to androgens, DHT binds to the androgen receptor in the follicle, gradually shortens the growth phase of the hair and leads to follicle miniaturization — hairs become thinner, shorter and more transparent until they are no longer visible. Blocking the enzyme reduces the amount of DHT and slows this process, and in some people partly reverses it.

There are at least two main isoenzymes of 5-alpha-reductase. Type II is found mainly in hair follicles (especially in the root sheath), the prostate and other genital tissues. Type I is present mainly in the sebaceous glands of the skin (including the scalp), in the liver and in some other tissues. Finasteride is a selective type II inhibitor, whereas dutasteride is a dual inhibitor that blocks both type I and type II. This explains its stronger effect on DHT concentrations in blood and in the scalp.

The key question is whether blocking type I also matters for hair. The authors of one of the first large comparative trials, described below, proposed that both isoenzymes may play a role in the pathogenesis of hair loss, which would justify the advantage of a dual drug. This remains inference from trial results rather than mechanistic proof at the level of a single follicle, and it deserves caution.

These figures explain where the popular ‘90% versus 70%’ comes from. Two caveats are in order, however. First, the study was done in men with prostatic enlargement and measured serum DHT, not DHT inside the hair follicle. Second, it compared finasteride 5 mg, not the 1 mg dose used for hair loss. The 1 mg dose also strongly inhibits type II and lowers serum DHT in roughly the same range, but this specific result does not apply directly to the regimen used for hair loss.

Variability matters as well: among people taking finasteride 5 mg the standard deviation was more than 18 percentage points, versus about 3 for dutasteride 0.5 mg. This means that with finasteride some people achieve clearly weaker DHT suppression than the average, which may partly explain why the response to finasteride is unsatisfactory in some patients and why dutasteride is sometimes considered as a second step.

This is the most important evidence on the topic, because it covers the right population (men with androgenetic alopecia), the right finasteride dose (1 mg) and the right dutasteride dose (0.5 mg, the same as in the capsule licensed for prostatic enlargement). The result is consistent with an earlier, smaller study by the same research group.

It is worth noting that the dutasteride dose that beat finasteride in the Olsen trial was 2.5 mg — five times more than the standard 0.5 mg capsule. Only the later, larger trial showed an advantage at 0.5 mg as well. This is a good example of why single results should not be treated as conclusive.

Two important limitations should be noted. First, the meta-analysis contained only three studies and 576 people in total. Second, all of them covered 24 weeks of treatment. The response to 5-alpha-reductase inhibitors develops over months, and a stable effect is usually judged after a year or more, so 24 weeks is a good signal but not an answer to the question of long-term efficacy.

Intuition suggests that a drug lowering DHT by ~95% should cause more adverse effects than one lowering it by ~70%. Randomized trials, however, do not show a clear difference: in the Gubelin Harcha trial the number and severity of adverse events were similar in the dutasteride, finasteride and placebo groups, and the Zhou meta-analysis found no significant difference in libido change between the drugs. At the same time, 24 weeks and a few hundred participants are too little to catch rare or delayed problems.

This result is sometimes cited as proof that 5-ARIs do not affect sexual function in men with hair loss. That is an over-interpretation: lack of statistical significance with wide confidence intervals means uncertainty, not absence of risk. The whole controversy around adverse effects (libido, mood, post-finasteride syndrome) is discussed separately in our article on finasteride side effects — here it is enough to say that there are no good data to claim that either of the two drugs is clearly safer in this respect.

An important practical difference between the drugs concerns pharmacokinetics. Finasteride has a short half-life, on the order of hours, and its action reverses within weeks of stopping. Dutasteride is distinguished by exceptionally slow elimination — in a classic pharmacokinetic study in healthy volunteers the half-life reached up to 5 weeks at high concentrations, shortening to about 3 days at low concentrations (non-linear elimination). Inter-individual variability in clearance was very high (coefficient of variation 70%).

What does this mean in practice? First, any adverse effects after stopping dutasteride may fade more slowly than after finasteride, and a full return to baseline DHT takes months. Second, according to the summary of product characteristics, dutasteride is associated with a longer period during which blood donation is not allowed (much longer than with finasteride), and women who are pregnant or may become pregnant should not handle leaking capsules. Third, the longer action is sometimes used as an argument for less frequent dosing, but such regimens have no solid comparative trials and should not be taken as a recommendation.

In Poland, as in most EU countries, dutasteride 0.5 mg (as capsules) is licensed for symptomatic benign prostatic hyperplasia and requires a prescription. Its use in androgenetic alopecia is outside the licensed indications (off-label). Dutasteride has, however, been approved for this indication in South Korea and Japan, which is one reason Asian clinical trials are available. Finasteride 1 mg is licensed for androgenetic alopecia in men in Poland.

Off-label use is neither illegal nor inappropriate by definition — a doctor can propose it after weighing benefits and risks, especially when finasteride does not work. It does mean, however, that the leaflet does not refer to this use, responsibility for the decision is shared, and the patient should be aware that assessment of long-term safety in hair loss relies on extrapolation from prostatic enlargement. In a comparative study in prostatic enlargement (EPICS, more than 1,600 men, 12 months), dutasteride 0.5 mg and finasteride 5 mg were equally effective in reducing prostate volume and symptoms, and the percentage of adverse events was similar.

A similar misunderstanding is the belief that suppressing DHT ‘to zero’ is a goal in itself. DHT has physiological functions in the skin, the genital organs and, according to preclinical research, in the brain (through neurosteroids formed with the participation of 5-alpha-reductases). So the question is not ‘how do I lower DHT as much as possible’ but ‘what degree of reduction gives a sufficient effect on the hair at an acceptable risk’.

From the standpoint of pharmacology and randomized trials, dutasteride is a stronger drug than finasteride 1 mg — and that is well documented. From the standpoint of clinical practice the question is different: is this additional effect worth going outside the licence, a longer elimination from the body and the lack of long-term data comparing the two drugs? For most people the answer is: finasteride first, assessment after a year, and dutasteride as a conscious decision with the doctor if the effect is insufficient. Whichever you choose, what matters most are realistic expectations, regular assessment of the effect and a quick reaction to worrying symptoms.