Editor's note: Vancocin is no longer patent-protected. An earlier version of this article stated otherwise. We regret the error.

Nothing is scarier for a pharmaceutical company than the potential threat of generic drug competition. For Pennsylvania-based ViroPharma (NASDAQ:VPHM), this threat is even more dire, because the company derives all of its revenues from one drug, Vancocin, that might be facing generic competition in the near future.

As one of my favorite Fools, Rich Duprey, reported earlier in the year, it's still uncertain exactly when Vancocin might face generic competition. Assuming the worst, it could be as soon as two years from now.

ViroPharma desperately needs a new drug from its pipeline as its only approved drug, Vancocin, could soon face generic competition, which would have a negative impact on sales. Fortunately, ViroPharma is on its way to ensuring that it will have such a drug candidate nearing approval if this worst-case situation happens for the company.

Yesterday, the company announced that it had started a phase 3 trial of a drug named Maribavir to prevent cytomegalovirus (CMV, a herpes virus) infection after stem cell transplantation. Patients with suppressed immune systems, such as those who have undergone a transplant, are at high risk for CMV infection, which can lead to serious complications or death. In the U.S. alone, this represents a potential market for Maribavir of more than 46,000 patients annually.

According to ViroPharma, it will take about 18 months to enroll all 500 patients in the first Maribavir phase 3 trial. With a 180-day follow-up period after patient enrollment, this puts the timeline for possible trial results around 28 months from now, according to my estimates. A second phase 3 trial of the drug is expected to launch later this year.

The timeline for Maribavir is still somewhat murky, but assuming the second phase 3 trial will run as long as the first one is expected to, that puts potential regulatory approval of the drug at no earlier than three years from now (assuming no clinical trial setbacks).

Earlier in the year, ViroPharma released preliminary results of a phase 2 trial for Maribavir. The efficacy results looked good and showed that no patients treated with the drug developed herpes, whereas 11% of the placebo patients did. The company released scant details of the drug's safety, but did note that Maribavir had a "favorable tolerability profile."

I'm always skeptical to take management's words on a drug's trial results until I can read more about them in a scientific journal or presentation. I won't make any final judgments on the prospects of Maribavir for approval until the drug's full phase 2 results are released during a medical conference in early December.

Investors should watch intently to make sure there are no trial enrollment delays or other setbacks for Maribavir. Regardless, with Maribavir now in the final stage of clinical trials, ViroPharma has come a long way toward staving off possible generic competition to its lead drug. That's something that should make any ViroPharma investor happy.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy .