A large portion of Gilead Sciences' (NASDAQ:GILD) future growth will come from its new hepatitis C drug Sovaldi, and drug cocktails that contain the drug, which are currently under review at the FDA. Here are three topics that investors should keep their eyes on.

Patient breakdown
On the earnings conference call earlier this month, management said that 70% of patients taking Sovaldi were infected with genotype 1 virus. That's in line with the 76% of hepatitis C patients in the U.S. that are infected with genotype 1, but it's a bit surprising because genotype 1 patients have to take Sovaldi with peg interferon, which has to be injected, and produces nasty flu-like side effects. With an all-oral cocktail for genotype 1 patients expected to be approved by year's end, you would expect that most patients with genotype 1 virus would wait.

The simple explanation is that the genotype 1 patients taking Sovaldi now are primarily advanced-stage patients who can't wait months to be rid of their virus. Depending on how large that group of warehoused patients is, we could see a slowing of sales as the bolus winds down before the all-oral cocktail is launched.

A delay of a few months capturing sales isn't that big of deal and is probably ideal, because the cocktail will likely be priced higher than the individual Sovaldi tablets. But AbbVie (NYSE:ABBV) is hot on Gilead's tail with its own all-oral cocktail, so Gilead isn't guaranteed to capture sales from patients who delay taking Sovaldi now.

Capturing the other 3.7 million
Gilead estimates that there are 4.1 million people in the U.S. infected with hepatitis C, but only 385,000 are under the care of a physician. Treating the rest of the patients is the only way Gilead can hit the astronomical sales estimates that investors are expecting and avoid an Incivek-like sales curve. Vertex Pharmaceuticals (NASDAQ:VRTX) made its hepatitis C drug, Incivek, an instant blockbuster; but because patients don't have to stay on the drug after they're cured, sales shot down just as quickly as new patients waited for Sovaldi.

About 1.7 million of the infected Americans actually know they're infected, so getting those patients treated shouldn't be that hard. How much they'll pay is another story. Hepatitis C patients are disproportionately low income, requiring discounted medications.

The more challenging group is the estimated 60% or so of patients who don't even know that they're infected. Having an effective cure available will hopefully encourage doctors to test a wider swath of people for the infection.

More than just hepatitis C
While hepatitis C is the new growth story, Gilead can't lighten up on the rest of its pipeline and marketed drugs. The biotech is the dominant force in HIV; 92% of new patients in the U.S. started on a Gilead HIV product in the third quarter of last year.

Importantly, sales growth is coming from the most ideal products. Sales of its new HIV drug Stribild continue to rise -- up over 400% year over year in the fourth quarter. Gilead owns all the components of Stribild, and doesn't have to share revenue like it does with Atripla, which Bristol-Myers Squibb (NYSE:BMY) helped develop. In the fourth quarter, sales of Atripla were up just 2% year over year.

Beyond HIV, Gilead is developing quite a few other drugs. Furthest along is a heart drug called ranolazine and momelotinib for myelofibrosis in phase 3 development, and idelalisib that's up for approval for two different blood cancers. In the best-case scenario, all three combined probably won't reach the sales that Stribild could produce, but they're worth keeping an eye on nonetheless.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.