Shares of Repros Therapeutics (NASDAQ:RPRX) jumped 76% today, after announcing positive clinical trial data.

But investors still aren't convinced that its testosterone-stimulating drug Androxal will be approved today. You can tell simply by the valuation -- $300 million -- they're assigning to the company.

The testosterone market is a $2-billion annually and growing opportunity, thanks to direct-to-consumer advertising that big pharma is putting into it. You've probably seen the commercials for AbbVie's Androgel, with the huge 1.62% being brought in with cranes. It doesn't get manlier than that.

The current testosterone boosters -- Androgel, Eli Lilly's Axiron, Endo Pharmaceuticals Fortesta, and Auxilium Pharmaceuticals' Testim -- are creams that have to be applied to the skin. Androgel's selling point is that it's more concentrated -- 1.62% versus 1% -- so men can apply less. But the area still has to be avoided by women and children, who could be exposed to the drug just by touching the skin.

Androxal, on the other hand, is taken orally, which would be a huge advantage. If approved, it's not hard to see how the drug could easily capture a quarter of the market, or $500 million annually. Even at a very conservative price/sales ratio of three, Repros is potentially a $1.5 billion company. That's five times where it is right now.

Investors are timid, because there's still one more phase 3 trial to go, and the trial announced today had some issues. The company discovered one site where the site administrator fabricated data. And that wasn't the site that had unusually low sperm counts disclosed back in January.

Repros says everything is fine. It's thrown out the fabricated data, and the unusually low sperm counts are likely due to increased sexual activity. The drug worked as it's supposed to, increasing testosterone levels, which increased the libido and, thus, decreased sperm counts.

It all sounds reasonable, but things that need to be explained tend to be frowned upon by the Food and Drug Administration. The agency turned down Johnson & Johnson's antibiotic ceftobiprole because the FDA deemed many of the clinical investigation sites unreliable.

A second positive trial without any complications will go a long way toward giving investors confidence that the drug can be approved by the FDA. Repros has a long way to run, but that's because it's still risky.