According to ViiV Healthcare -- GlaxoSmithKline (NYSE:GSK), Pfizer (NYSE:PFE), and Shionogi's HIV drug consortium -- one in three Americans fails to take their HIV medication as prescribed. That means hundreds of thousands of patients aren't receiving the medicine necessary to delay the progression of HIV and the onset of AIDS.

That creates a significant hurdle for policy makers eager to lower HIV incidence rates and drugmakers eager to boost prescription volume for next-generation therapies.

As a result, drugmakers are innovating HIV treatment by creating multi-drug, single-tablet therapies designed to make treatment regimens easier on patients.

Source: ViiV Healthcare.

The latest of these combination therapies to win FDA approval is ViiV's Triumeq, a medicine that consists of ViiV's recently approved Tivicay and two older drugs: abacavir and lamivudine.

ViiV hopes that this multi-drug combination can not only improve patient adherence rates, but win away market share from HIV drug treatment leader Gilead Sciences (NASDAQ:GILD), which markets some of the most widely prescribed therapies for the disease, including Atripla.

Big, unmet need
Despite significant advances in treating HIV, the disease remains widespread. Over the past 20 years, innovative treatment and earlier identification have led to patients' average lifespan following HIV diagnosis more than doubling -- to 22 years.

Although that marks an impressive improvement, a lot of work remains to be done.

According to the World Health Organization, more than 1.5 million people died from HIV globally in 2012, and more than 33 million remain infected with the disease worldwide.

That includes more than 1.1 million patients here in the United States, roughly 18% of who haven't yet been diagnosed with the disease.

And while significant energy has been spent educating at-risk populations about HIV and prevention, about 50,000 new cases are being diagnosed in the U.S. every year.

Source: Express Scripts

Establishing leadership
A substantially under-diagnosed patient population means drugmakers have plenty of incentive to develop new medicines for the indication. 

The HIV treatment market is already valued at $18 billion, making it the fourth-largest specialty drug market in terms of spending, according to Express Scripts

A host of companies have launched successful HIV therapies in a bid to capture that market, but none has been more successful than Gilead.

Gilead's HIV drugs generated more than $9 billion in sales last year, and its HIV line-up includes five medicines likely to achieve billion-dollar blockbuster status this year, including Atripla, a combination therapy that consists of Gilead's Viread and Emtriva and Bristol-Myers' Sustiva.

Gilead sold more than $3.6 billion worth of Atripla last year, making it the globe's most widely used HIV therapy, but ViiV thinks it may have a shot at winning away some of Atripla's patients.

That's because Triumeq was statistically better than Atripla at controlling patient's viral load during clinical trials. At 96 weeks, 80% of participants on the Triumeq regimen were virologically suppressed compared to 72% of participants receiving Atripla. If that performance is convincing enough, doctors may begin prescribing Triumeq instead of Atripla.

A wave of new therapies
ViiV's Triumeq may have put up impressive results versus Atripla during trials, but Gilead has already launched Complera and Stribild -- two combination drugs well on their way to becoming top sellers.

Complera, which combines Gilead's Truvada with Johnson & Johnson's Edurant, costs $20,000 per year and had sales of nearly $300 million in the second quarter, while Stribild combines Gilead's Vitekta, Tybost, Viread, and Emtriva in one pill, costs $28,000 per year, and had sales of $270 million in the second quarter.

Fool-worthy final thoughts
Express Scripts estimates that spending on HIV medicine will climb 15.9% next year and another 13.1% in 2016 as healthcare insurance reform, including Medicaid expansion, boosts testing and prescriptions.

Whether ViiV's newly approved Triumeq will win away patients from Atripla or Stribild is anyone's guess, but it may not be a zero-sum game. With HIV drug demand on the rise, its possible there will be room for both Gilead's and ViiV's therapies.