BOSTON -- Women aren't the only ones suffering through "hormonal difficulties" as they get older.

It happens to guys, too. As men get older, testosterone production declines, leading to a host of symptoms that include reduced sex drive, erectile dysfunction, loss of muscle and energy, even depression.

The medical term for this condition is hypogonadism, but it can also be known as male menopause. (Or how about "manopause?") Researchers believe that 19 million men in the U.S. suffer from testosterone deficiency, but right now, only 10% of them are diagnosed and treated, according to a report compiled by ThinkEquity Partners, which held a conference on reproductive health (including male sexual health issues) last week in New York.

Still, this relatively small patient population accounts for about $500 million to $600 million in annual sales of testosterone replacement products -- gels, patches and injectables. And as U.S. males get older and more of them seek out diagnosis and treatment, this testosterone replacement market is going to grow strongly.

With that in mind, here are three "male menopause" stock plays to consider:

Indevus Pharmaceuticals ( IDEV):

The current testosterone replacement market is dominated by quick-drying gels, which men must rub on their bodies (usually a shoulder) daily. The gels become part of a daily routine, but still, the hassle of applying a gel to the body every day, waiting for it to dry before dressing, etc., can be a chore.

Likewise, current testosterone shots are too short-acting, requiring men to inject themselves weekly or bi-weekly. These products can also cause excessive spikes in testosterone levels, leading to unwanted side effects.

Indevus' answer to these problems is Nebido, a long-acting testosterone injection that lasts three months and doesn't cause the nasty hormonal spikes of current injectables.

Nebido was filed with the Food and Drug Administration at the end of August. The agency should decide on the drug's approval by the end of next summer.

With just four shots a year, Nebido would allow men to treat their hypogonadism without worrying about fluctuating testosterone levels and freeing them up from having to apply a daily gel.

At the ThinkEquity conference, Indevus estimated there are about 130,000 men using current testosterone injections. When Nebido is approved, these men should be easily switched over.

After that, Indevus plans to target the approximately 210,000 men currently using gels or patches. The company doesn't expect to convert a majority of gel users, but there will be a significant number of men who prefer four easy injections per year.

Nebido is already approved and used in Europe, where it is marketed by Bayer Schering without any significant safety issues. One concern raised at last week's conference, however, was the fact that a long-acting testosterone therapy might prove problematic for men who are found later to have rising PSA levels, a danger sign for prostate cancer. (Testosterone "feeds" prostate cancer cells, which is why prostate cancer patients usually go on testosterone-dampening therapy.)

Indevus was recently off 3 cents to $7.09.

Repros Therapeutics ( RPRX):

Testosterone replacement products do exactly what you'd think. One thing they can't do, however, is treat the underlying cause of testosterone deficiency. In other words, they can't help the body produce any more natural, or endogenous, testosterone.

Repros is tackling this issue with its product Androxal. A short-acting form of the commonly used female fertility drug Clomid, Androxal works by stimulating the pituitary gland to increase production of two hormones that, in turn, cause the testes to ramp up production of testosterone.

Androxal has the advantage of treating the underlying condition that causes lower testosterone levels. Androxal therapy also that it raises sperm count, whereas testosterone replacement products actually reduce it.

Data collected so far on Androxal has shown that the drug works as advertised, but that hasn't stopped Repros from hitting some snags. The FDA has raised the bar on approval for new male hypogonadism products. It's no longer sufficient to just show an increase of testosterone levels to a normal range; now the FDA is asking for quality-of-life data that shows improvement in symptoms associated with lower testosterone.

Repros plans to meet with FDA officials later this month to discuss this new requirement and figure out what specific data it can generate on Androxal to get the product approved for use.

Repros was up 7 cents to $11.36 in recent trading.

Auxillium Pharmaceuticals ( AUXL):

Auxillium has performed very well with Testim, a once-daily testosterone gel that is grabbing steady market share from market leader Androgel, sold by Solvay. Auxillium has benefited from clinical data that shows Testim delivers more testosterone than Androgel.

Since launching in 2003, Testim sales have racked up a compound annual growth rate of almost 75%. As of August, Testim's share of the testosterone gel market was 20.5% and growing. Sales in the first half of 2007 totaled $42 million, which puts the company on track to meet its projected sales guidance of $88 million to $92 million for the year.

While Auxillim should continue to increase Testim's market share, just as importantly, the product creates a solid base business that the company can use to invest in other pipeline drugs. Most notable is Xiaflex, an enzyme that breaks down collagen and is being developed to treat three different diseases in which collagen builds up abnormally to create painful and movement-restricting nodules under the skin.

Auxillium was up 3 cents to $23.52.
Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.