What to Know About 5-Alpha Reductase Inhibitors

Medically Reviewed by Nazia Q Bandukwala, DO on July 18, 2023
4 min read

The class of medicines called 5-alpha reductase inhibitors (5ARIs) includes finasteride and dutasteride. These medicines treat benign prostatic hypertrophy and male pattern hair loss. There is a concern that 5ARIs may increase the risk of developing a serious form of prostate cancer.

Finasteride and dutasteride are approved to treat benign prostatic hypertrophy and male pattern hair loss.

Benign prostatic hypertrophy (BPH). This condition is also referred to as an enlarged prostate. It's a common condition that occurs as men get older. Your prostate gland is about the size of a walnut and is located between your penis and rectum. It produces your seminal fluid, which allows sperm from your testes to travel and survive.

Your urethra is a narrow tube that connects to your bladder and allows urine and semen to exit your body. The urethra runs through the middle of your prostate. If your prostate is enlarged, it can block your flow of urine and cause bladder, urinary tract, and kidney problems. Symptoms of BPH usually get worse over time and may include:

  • Needing to urinate frequently
  • Feeling an urgent need to urinate
  • Weak urine stream
  • Urine stream that stops and starts
  • Being unable to completely empty your bladder
  • Needing to urinate frequently at night
  • Dribbling at the end of urination

Male pattern hair loss. About half of all men will lose some hair by the time they're 50. Male pattern hair loss, a condition called androgenetic alopecia (AGA), is genetic. Hair loss typically begins at the temples, the front of your head, and the crown of your head. Eventually, the bald areas spread and you're only left with hair below your ears.

AGA is caused by a combination of your hormones and your genes. Androgens are hormones that are primarily responsible for male traits. If you have the genetic trait for AGA, the androgens you produce cause your hair follicles to get smaller. When your hair falls out, it's replaced by small, colorless hairs. Over time, your follicles stop producing any hair. 

The enzyme 5-alpha reductase is present in your liver, skin, and prostate. This enzyme converts some of the testosterone you produce into dihydrotestosterone (DHT). DHT plays a role in prostate growth, acne, growth of facial hair, and male pattern baldness. The 5ARIs work by suppressing the 5-alpha reductase enzyme. This decreases the amount of DHT your body produces.

Both finasteride and dutasteride inhibit the 5-alpha reductase enzyme. This enzyme has two smaller parts, called isoenzymes. Type 1 is found mainly in the liver and skin, with smaller amounts in the prostate. Type 2 is found mainly in the prostate, with a little bit found in the skin and liver.

Finasteride. Finasteride works by inhibiting only the Type 2 isoenzyme of 5AR. In a review of clinical trials with both medicines, finasteride was found to reduce DHT production by 70%. It resulted in an 18% decrease in prostate size. Finasteride doesn't stay in your system as long as dutasteride does.

Finasteride is the only 5ARI approved for use in male pattern hair loss. Finasteride is available as a tablet in 1-milligram and 5-milligram dosages. It may interact with St. John's Wort, an herbal supplement. For hair loss, it will take at least 3 months to see an effect with finasteride. For BPH, it will take up to 6 months to see the full effect. For both hair loss and BPH, the effects will only last as long as you take the medicine.

Dutasteride. Dutasteride works by inhibiting both the Type 1 and Type 2 isoenzymes of 5-alpha reductase. In clinical trials, dutasteride was found to reduce DHT production by 93% and decrease the prostate size by 25.7%. The negative side effects of dutasteride were similar to those with finasteride. For treatment of BPH, dutasteride was about as effective as finasteride. 

Although it's not approved to treat male pattern hair loss, doctors may choose to prescribe it for hair loss. In a review of clinical trials using both finasteride and dutasteride for hair loss, dutasteride was more effective. Both medicines had similar side effects and negative reactions.

Both finasteride and dutasteride have similar side effects. Both medicines have been linked to slightly increased rates of: