Understanding 5-Alpha Reductase Inhibitors

Dealing with hair loss is unavoidable for many men. The good news is that there are many ways to protect your hairline, including medications known as 5-alpha reductase inhibitors (5ARIs).

These medications have been shown in placebo-controlled clinical studies to reduce the incidence of hair loss and treat enlarged prostates.

Keep reading for a comprehensive guide to 5-alpha reductase inhibitors, including how these drugs work, common side effects, and potentially harmful interactions.

5-alpha reductase inhibitors are a group of drugs used to treat male pattern hair loss, or androgenetic alopecia. They are also used for the management and treatment of an enlarged prostate gland, a condition called benign prostatic hyperplasia (BPH).

Drugs in this class include finasteride (sold under the brand names Proscar® and Propecia®) and dutasteride (marketed as Avodart®).

These drugs became available in 1992 when the Food and Drug Administration (FDA) approved finasteride.

In addition to hair loss and BPH, 5-alpha reductase inhibitors are occasionally used to treat hyperandrogenism (excessive testosterone) and hirsutism, which is excessive, abnormal growth of hair, typically in women. They may also be used for hormonal treatment in transgender women.

5-alpha reductase inhibitors work by blocking or inhibiting 5-alpha reductase — an enzyme that promotes the conversion of testosterone into dihydrotestosterone (DHT).

You need DHT and other forms of testosterone throughout your life. Testosterone is responsible for growth during puberty and increased muscle mass in men. DHT, on the other hand, is linked to male sex characteristics during prenatal development, prostate growth and hair patterns, including male pattern baldness.

In someone with benign prostate hyperplasia (BPH), DHT can build up in the prostate gland and bind to prostatic androgen receptors, leading to enlargement of the prostate. An enlarged prostate is a common problem that affects up to 50 percent of men by the age of 60, and the prevalence increases with age.

By blocking the conversion of testosterone into DHT, 5-alpha reductase inhibitors can decrease your prostate size.

Meanwhile, when DHT binds to androgen receptors in the scalp, it gradually causes hair follicles to stop producing new hair.

So, for those genetically predisposed to male pattern baldness, DHT inhibition is one of the primary ways to  protect hair follicles.

Our guide to DHT and male pattern baldness goes into more detail on this process and the effects of DHT on hair loss.

There are currently two FDA-approved 5-alpha-reductase inhibitors: finasteride and dutasteride. The primary difference is that one (finasteride) is commonly prescribed for hair loss, while the other (dutasteride) is prescribed for enlarged prostate.

Finasteride (Propecia and Proscar)

Finasteride is marketed under two different brand names — Propecia and Proscar — and sold as a generic medication.

It was developed in the 1970s and approved by the US Food and Drug Administration (FDA) in 1992 as a treatment for benign prostatic hyperplasia as Proscar. Eventually, finasteride was approved in 1997 as a treatment for hair loss in men under the brand name Propecia.

Depending on the condition you’re treating, finasteride is prescribed in different dosages as a daily oral tablet — 1mg of Propecia® is taken for hair loss treatment, while 5mg of Proscar® is taken for BPH.

Dutasteride (Avodart)

Dutasteride was patented in 1996 and received FDA approval in 2001 under the brand name Avodart.  It comes as a 0.5mg oral capsule taken once a day.

This drug helps manage symptoms of benign prostatic hyperplasia, which can include frequent urination, difficulty urinating, or pain during urination.

Dutasteride is sometimes prescribed as a combination pill known as Jalyn®, which also includes tamsulosin. Tamsulosin — also known by its brand name Flomax® — is an alpha-blocker medication that helps relax the muscles of the prostate and open the bladder to improve the symptoms of BPH.

Although some clinical trials and urology research show that dutasteride effectively treats hair loss, the FDA has only approved it to treat BPH.

If you want to know more about the effectiveness of dutasteride compared to finasteride for hair loss, check out our guide comparing the two medications.

Systematic review and meta-analysis data have repeatedly shown that 5-alpha reductase inhibitors are safe to use and effective. However, like any medication, they also have the potential to cause side effects. 

Finasteride and dutasteride have similar possible adverse effects, including:

• Sexual dysfunction

• Erectile dysfunction

• Decreased libido

• Gynecomastia (enlargement of breast tissue in men)

• Depression

• Anxiety

• Increased risk of high-grade prostate cancer

Jalyn® has also been reported to cause dizziness, weakness, and orthostatic hypotension, which is low blood pressure that occurs when standing up from a lying or sitting position.

5-alpha reductase inhibitors also come with potential risks and harmful interactions.

One is that the use of 5-ARIs can lead to an increased risk of prostate cancer.

Some men may experience a change in prostate-specific antigen (PSA) levels while taking a 5-alpha reductase inhibitor. An increase in PSA levels while on treatment with a 5-alpha reductase inhibitor may indicate the presence of prostate cancer.

PSA levels should be measured before starting a 5-alpha reductase inhibitor and monitored throughout treatment to evaluate the drug’s effects.

Finasteride may interact with St. John's Wort, an herbal supplement used for depression, anxiety, attention-deficit hyperactivity disorder, and other conditions.

If you notice any of these side effects or symptoms of a drug interaction, contact your health provider immediately.

While hair loss may be a common problem for many men, treatment can restore hair growth. One example of a common hair loss treatment is 5-alpha reductase inhibitors.

Two examples of 5-alpha reductase inhibitors are finasteride and dutasteride. Here are the basics you should remember about them:

• Both work by inhibiting the 5-alpha reductase enzyme and stopping your body from converting testosterone into dihydrotestosterone, or DHT. 

• By reducing DHT levels, both medications can slow down, stop, or reverse the effects of male pattern baldness.

• This class of drugs can also treat an enlarged prostate and help manage the symptoms of benign prostatic hyperplasia.

If you’re concerned about hair loss or an enlarged prostate, it’s important to talk to a healthcare provider to find the best treatment plan for you — which may be a 5-alpha reductase inhibitor.