2. Biktarvy

Biktarvy has a peak sales estimate of $6 billion, according to analysts. (Gilead Sciences)

Company: Gilead Sciences
Used for: HIV
2019 sales: $4.74 billion

When Gilead Sciences' next-gen HIV therapy Biktarvy was approved in early 2018, analysts pegged it as a megablockbuster right out of the gate. There was one catch: Gilead would have to compete with GlaxoSmithKline and its two-drug combo challengers, but so far so good. And in 2020—at the least—Biktarvy is expected to add to its war chest. 

Biktarvy's sales could increase by $1.6 billion in 2020, taking its 2019 haul of $4.74 billion up to roughly $5.34 billion, according to EvaluatePharma. 

To reach its $6 billion in peak sales estimate, Gilead had a simple plan for Biktarvy, a three-drug combination of novel integrase inhibitor bictegravir with Gilead's Descovy (emtricitabine and tenofovir alafenamide): Target new patients and switch patients over from older Gilead drugs.

Just two years into the launch, that approach has worked wonders. Biktarvy scored more than $1.5 billion in sales in the fourth quarter alone—blowing away Street estimates of $1.2 billion. As of March, Biktarvy had reached nearly 2 million total prescriptions, according to RBC Capital Markets analyst Brian Abrahams, with 4.8% growth week over week. 

On the whole, Gilead's entire HIV portfolio hit $16.4 billion in sales in 2018. 

RELATED: GlaxoSmithKline's next-generation HIV battle against Gilead is already taking a toll on growth

2. Biktarvy