What Class of Drug Is Finasteride? A Comprehensive Look at 5-Alpha Reductase Inhibitors

October 8, 2025 •

4 min read

First approved by the FDA in the 1990s, finasteride became a standard treatment for male pattern hair loss and enlarged prostate by targeting a key hormone. But what class of drug is finasteride, and how does it achieve these therapeutic effects?

Quick Summary

Finasteride is a medication classified as a 5-alpha reductase inhibitor, targeting the enzyme that converts testosterone into the potent androgen dihydrotestosterone (DHT). It is medically approved for treating male pattern baldness and benign prostatic hyperplasia (BPH) by lowering DHT levels in the body.

Key Points

Drug Class: Finasteride is a 5-alpha reductase inhibitor, meaning it blocks the enzyme that converts testosterone into dihydrotestosterone (DHT).
Mechanism of Action: By inhibiting the Type II 5-alpha reductase enzyme, finasteride lowers systemic and local DHT levels, addressing conditions caused by excess DHT.
Therapeutic Uses: It is used to treat both male pattern hair loss and benign prostatic hyperplasia (BPH).
Side Effects: Potential side effects include decreased libido, erectile dysfunction, and, in rare cases, depression and breast tissue changes.
Treatment Comparison: Alternatives like dutasteride offer more complete DHT reduction, while minoxidil uses a different mechanism to stimulate hair growth.

What Is a 5-Alpha Reductase Inhibitor?

Finasteride belongs to the class of medications known as 5-alpha reductase inhibitors (5-ARIs). The function of this drug class is to block the action of the 5-alpha reductase enzyme, which plays a crucial role in converting the hormone testosterone into a more potent androgen called dihydrotestosterone (DHT). By inhibiting this enzyme, finasteride effectively lowers DHT levels in both the blood and specific tissues, such as the prostate and scalp. This targeted hormonal modulation is key to its therapeutic effects in treating conditions driven by an excess of DHT.

There are two main isoenzymes of 5-alpha reductase in the human body: Type I and Type II. Finasteride is a competitive and specific inhibitor of the Type II isoenzyme, which is responsible for the majority of circulating DHT. Its targeted action explains why it is effective against conditions like male pattern baldness and BPH, where the Type II isoenzyme is particularly active. Another prominent 5-ARI, dutasteride, inhibits both Type I and Type II isoenzymes, leading to a more complete reduction of DHT.

The Mechanism of Action and Therapeutic Uses

The underlying principle of finasteride's mechanism is the reduction of DHT, which is implicated in several male health conditions. The drug's efficacy differs depending on the condition being treated:

Male Pattern Hair Loss (Androgenetic Alopecia): Finasteride is used to treat male pattern hair loss. In men with this condition, elevated levels of DHT on the scalp lead to the miniaturization of hair follicles, resulting in a shortened hair growth cycle and gradual hair loss. By blocking DHT production, finasteride can slow down or even reverse this process, promoting hair regrowth. Noticeable improvements typically take several months to appear, and the medication must be taken continuously to maintain the effects.
Benign Prostatic Hyperplasia (BPH): Finasteride is used to treat BPH, or an enlarged prostate gland. In BPH, DHT causes the prostate gland to grow larger, which can lead to bothersome urinary symptoms like frequent urination, difficulty starting urination, and a weak stream. Finasteride helps shrink the enlarged prostate by blocking the effect of DHT, which can alleviate these symptoms and reduce the risk of needing surgery. It can take up to six months to see the full effect of the medication for BPH.

Potential Side Effects and Considerations

As with any medication, finasteride carries potential side effects, and patients should discuss the risks and benefits with their healthcare provider.

Common side effects associated with finasteride include:

Decreased libido (sexual desire)
Erectile dysfunction
Ejaculation problems, including decreased volume

Serious, though rare, side effects that warrant immediate medical attention include:

Changes in breast tissue, such as enlargement, pain, or lumps
Allergic reactions, such as rash, itching, or swelling of the face or lips
Depression and suicidal thoughts have also been reported in some users

Finasteride is also known to affect prostate-specific antigen (PSA) blood test results, which are used to screen for prostate cancer. It is important to inform your doctor you are taking finasteride before a PSA test. Studies have shown a reduced overall risk of low-grade prostate cancer with finasteride use, but a potential for an increased risk of high-grade prostate cancer has been a topic of discussion.

Comparison of Finasteride and Alternatives

Finasteride is not the only option available for hair loss or BPH. Below is a comparison of finasteride with other common treatments for these conditions.


Feature	Finasteride (e.g., Propecia, Proscar)	Dutasteride (e.g., Avodart)	Minoxidil (e.g., Rogaine)
Drug Class	5-Alpha Reductase Inhibitor (Type II)	5-Alpha Reductase Inhibitor (Type I & II)	Vasodilator	n	Mechanism of Action	Blocks Type II 5-alpha reductase, reducing DHT	Blocks Type I and II 5-alpha reductase, reducing DHT	Increases blood flow to hair follicles, prolonging the anagen phase	n	Primary Use(s)	Male pattern hair loss, BPH	BPH, off-label for hair loss	Male pattern baldness	n	Administration	Oral tablets	Oral capsules	Topical solution or foam	n	FDA Approval for Hair Loss	Yes (Propecia)	No (off-label use)	Yes (Rogaine)
Efficacy	Effective for most men with hair loss or BPH	May be more potent for severe hair loss due to blocking both isoenzymes	Works well for crown thinning; can be combined with finasteride
Side Effect Profile	Includes sexual dysfunction, depression	Similar to finasteride but potentially higher risk of side effects due to more complete DHT blocking	Primarily localized scalp irritation, less systemic side effects

The Importance of Medical Consultation

Choosing the right treatment for conditions like hair loss or BPH is a personal decision that should be made with a healthcare professional. Your doctor can help determine the most suitable option based on your specific condition, medical history, and risk tolerance. It is crucial to understand the mechanism of action, therapeutic applications, and potential side effects of any medication you consider.

For more detailed, authoritative information, you can visit the NIH StatPearls entry on Finasteride.

Conclusion

In summary, finasteride is a well-established member of the 5-alpha reductase inhibitor class of drugs. Its primary action is to reduce levels of the androgen DHT, which is a key hormonal driver in both male pattern baldness and benign prostatic hyperplasia. The drug's dual application demonstrates its targeted action on tissues sensitive to DHT. While effective, it is associated with potential side effects, particularly relating to sexual function, and requires ongoing use to maintain its benefits. Always consult a healthcare provider to determine if finasteride is the right treatment for you.

Frequently Asked Questions

Both finasteride and dutasteride are 5-alpha reductase inhibitors. However, finasteride primarily inhibits the Type II isoenzyme, while dutasteride inhibits both Type I and Type II isoenzymes, leading to a more complete reduction of DHT.

No, finasteride is not approved for use in women, especially those who are pregnant or may become pregnant, due to the risk of birth defects in a male fetus. Other treatments may be available for female hair loss.

For male pattern hair loss, it may take at least three months to see improvement, with noticeable results within 12 months. For BPH, symptom improvement can take up to six months.

The effects of finasteride will reverse if you stop taking it. For hair loss, any hair that grew back will likely be lost within 12 months. For BPH, the prostate gland will begin to grow again.

Finasteride does not significantly lower overall testosterone levels. By preventing its conversion to DHT, it may cause a slight increase in circulating testosterone.

Yes, Propecia is indicated for male pattern hair loss, while Proscar is indicated for benign prostatic hyperplasia (BPH). They are not interchangeable, and your doctor will prescribe the correct medication for your condition.

Yes, finasteride can decrease PSA levels, which are used to screen for prostate cancer. You should inform your doctor you are taking finasteride so they can adjust the interpretation of your PSA results.