Medically reviewed by Kristin Hall, FNP
Written by Our Editorial Team
Last updated 8/21/2022
Dealing with hair loss can be unavoidable for many men. Whether your hairline is only starting to recede or thinning all over, hair loss isn’t very enjoyable. You don’t have to sit back and watch your hair disappear, though. Today, there are many ways to protect your hairline, from medications to procedures like hair transplantation. One group of medications that treat hair loss is known as 5 alpha reductase inhibitors (5ARIs).
These drugs are present in several men’s health medications so we’ve covered everything you’d want to know about 5-alpha reductase inhibitors.
From how these drugs work and common side effects to potentially harmful interactions, keep reading for a comprehensive guide to 5 alpha reductase inhibitors.
5-alpha reductase inhibitors are a group of drugs used in the treatment of male pattern hair loss, or androgenetic alopecia.
These medications are also used for the management and treatment of an enlarged prostate gland (benign prostatic hyperplasia).
Drugs in this class include finasteride (sold under the brand names Proscar® and Propecia®) and dutasteride (marketed as Avodart®).
These drugs first became available in 1992, after finasteride’s approval by the Food and Drugs Administration (FDA).
Among hair loss and enlarged prostate glands, alpha reductase inhibitors may also be used to treat:
Hyperandrogenism (excessive presence of testosterone)
Hirsutism (excessive, abnormal growth of hair, typically in women)
Hormonal treatment in transgender women
Alpha reductase inhibitors work to reduce hair loss and thinning as well as treat enlarged prostate glands.
5-alpha reductase inhibitors work by blocking 5-alpha reductase, an enzyme that promotes the conversion of testosterone into dihydrotestosterone (DHT).
Testosterone converts into DHT in tissue such as the skin, liver, prostate and hair follicles.
While testosterone and DHT are both male sex hormones that bind to androgen receptors throughout the body, they have different effects.
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.
For those genetically predisposed to male pattern baldness, DHT binds to androgen receptors in the scalp and causes hair follicles to stop producing new hairs gradually.
Our guide to DHT and male pattern baldness goes into more detail on this process and the effects of DHT on hair loss.
DHT is also responsible for prostate growth. An enlarged prostate is a common problem that affects up to 50 percent of men by the age of 60 and increases with age.
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.
5-alpha reductase inhibitors block the enzyme responsible for testosterone converting to DHT, causing a decreased prostate size.
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There are currently two FDA-approved uses for 5-alpha-reductase inhibitors: managing hair loss and BPH.
Some healthcare providers may use alpha reductase inhibitors “off-label” for the treatment of hirsutism, a condition of excessive hair growth, typically in women.
The two FDA-approved 5-alpha reductase inhibitor drugs are finasteride and dutasteride.
Finasteride is marketed under two different brand names: Propecia® and Proscar®.
Finasteride was originally developed in the 1970s and approved by the FDA in 1992 as a treatment for benign prostatic hyperplasia as Proscar®.
Finasteride was approved in 1997 as a treatment for hair loss in men under the brand name Propecia®.
Both drugs are used to treat BPH and its symptoms, such as frequent and difficult urination. It can also decrease the possibility of future prostate surgery.
Finasteride is also used to treat male pattern hair loss, gradual thinning of the hair on the scalp, a receding hairline or balding.
Depending on the condition being treated, 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 was patented in 1996 and received FDA approval in 2001 under the brand name Avodart®.
These drugs help manage symptoms of benign prostatic hyperplasia, which can include frequent urination, difficulty urinating or pain during urination.
Although some research shows that dutasteride is effective at treating hair loss, it is only approved by the FDA to treat BPH.
The effectiveness of dutasteride compared to finasteride is explored in more detail in our guide comparing the two medications.
Dutasteride comes as a 0.5mg oral capsule taken once a day.
Dutasteride is sometimes prescribed as a combination pill known as Jalyn® that 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 opening of the bladder to help improve the symptoms of BPH.
Like any medication, 5-alpha reductase inhibitors do come with the potential for side effects.
Both finasteride and dutasteride have similar possible side effects including:
Gynecomastia (enlargement of breast tissue in men)
Increased risk of high-grade prostate cancer
Jalyn® has been reported to cause orthostatic hypotension, or low blood pressure, that occurs when standing up from a lying or sitting position, dizziness and weakness.
Alpha reductase inhibitors do come with potential risks and harmful interactions.
The use of 5-alpha reductase inhibitors can lead to an increased risk of high-grade prostate cancer.
Some men may experience an increase or decrease 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 the use of alpha reductase inhibitors and monitored throughout treatment to evaluate the effects of the drug.
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 is possible. One example of a common hair loss treatment is 5 alpha reductase inhibitors.
Two examples of alpha reductase inhibitors are finasteride and dutasteride. 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 are shown to 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 about possible treatment plans.