Antiandrogens block DHT already produced and present in the blood stream from binding with hair follicles. Their specificity varies greatly from specific antiandrogens such as finasteride which inhibit the conversion of testosterone to DHT by interfering with 5-alpha-reductase to more broad spectrum antiandrogens (fluconazole, spironolactone, etc.) Although unusual in clinical doses, antiandrogens can have serious side effects including gynecomastia. Ketoconazole, (often sold as Nizoral Shampoo) and is prescribed by medical professionals or available over the counter depending on the product, concentration and country.

Finasteride

Main article: Finasteride

Finasteride, marketed as the brand-name drug Propecia and Proscar by Merck, belongs to a class of drugs called aza-steroids. Finasteride is a "DHT inhibitor" and was originally approved by the FDA for the treatment of benign prostatic hyperplasia (BPH). It accomplishes this by blocking the production of 5-alpha-reductase, the enzyme responsible for the conversion of free testosterone to DHT. Propecia (1 mg of finasteride daily) blocks approximately 55% of DHT activity and Proscar (5mg of finasteride daily) blocks 70%. Soon after its release, it was discovered that patients who were taking finasteride were experiencing hair regrowth. In 1997, Finasteride was approved by the FDA for the treatment of male pattern baldness. A 5 year study revealed that 9 of 10 men taking finasteride (1mg daily) experienced visible results (42% of men taking Propecia experienced no further hair loss while 48% experienced no further hair loss and hair regrowth). In clinical studies, finasteride, like Minoxidil, was shown to work on both the Crown (anatomy) area and the hairline area, but is most successful in the crown area.

Finasteride is usually only prescribed for men and should not even be touched by pregnant or potentially pregnant women, as it has been speculated that it could cause severe birth defects in male fetuses. Studies have shown that finasteride is ineffective for treating hair loss in women. However, finasteride's supporters respond that the study was on post-menopausal women whose hairloss was more likely related to the loss of estrogen versus a sensitivity to testosterone. Other studies have shown that finasteride is effective for many women with follicular sensitivity to androgens. Some doctors are now willing to prescribe finasteride to women on the condition that either the women is taking careful birth control measures or that the woman cannot become pregnant.

Dutasteride

Main article: dutasteride

In 2001, GlaxoSmithKline released another aza-steroid called dutasteride. Dutasteride is marketed as Avodart. Like finasteride, dutasteride was originally developed for the treatment of benign prostatic hyperplasia (BPH). While hair count studies showed that 2.5 mg of dutasteride was about 1.5 times as effective as finasteride for hair regrowth (adding on average 108 versus 72 hair per 1" diameter area), Glaxo stopped FDA hair loss studies after phase II. Although the exact reason was never made public, it was speculated that the product was too similar to finasteride, which itself had not lived up to expectations commercially. As such, the 2.5 mg dosage was not released. The FDA trials for BPH continued, and Avodart became the first drug shown to shrink an enlarged prostate in a clinical study. The .5mg version of the drug (shown in the same study to add on average 92 hairs to the same area) is increasingly available to hair loss sufferers via the grey-market of online prescription medication, and physicians increasingly willing to prescribe drugs "off-label."

In December 2006, GlaxoSmithKline embarked on a new Phase III, six month study in Korea to test the safety, tolerability and effectiveness of a once-daily dose of dutasteride (0.5mg) for the treatment of male pattern baldness in the vertex region of the scalp (types IIIv, IV and V on the Hamilton-Norwood scale). The future impact that this study will have on the FDA's approval or disapproval of Avodart for the treatment of male pattern baldness in the United States is yet to be determined.

Ketoconazole

Main article: Ketoconazole

Ketoconazole is a synthetic antifungal drug used to prevent and treat skin and fungal infections, especially in immunocompromised patients such as those with AIDS. Because it is both an anti-fungal and also a 5-alpha reductase inhibitor, it can help to slow the balding process. There has been some suggestion that ketoconazole could inhibit testosterone synthesis in utero, which could potentially inhibit genital development of a male fetus. However, this has not been documented in any controlled studies.

Saw palmetto

Main article: Saw Palmetto

Saw palmetto (Serenoa repens) is an herbal DHT inhibitor which is cheaper than most commercial drugs and is claimed to have fewer side effects than finasteride and dutasteride. Unlike other 5-alpha-reductase inhibitors, saw palmetto extract inhibits the conversion of testosterone to DHT without interfering with the cellular capacity to secrete PSA. Saw palmetto extract has been demonstrated to inhibit both isoforms of alpha-5-reductase, unlike finasteride which only inhibits the (predominant) type 2 isoenzyme of alpha-5-reductase. It must be noted that DHT inhibition in vitro does not necessarily imply inhibition in vivo. A preliminary study of saw palmetto extract for treating hair loss noted improvement in six of ten subjects over a six month period. Dosages were not reported. The small size of the study and its relatively short duration limit must be noted.

There hasn't been a single reputable study done showing that Saw Palmetto has any effect on treating hair loss. One study can be found in PubMed, but it was run by the makers of a saw palmetto product, and was publicly labeled as "Bunk" on 20/20 in January 2003 on nationwide television because it only included 10 participants and lasted only 6 months.