The story of Viagra (also known by its chemical name sildenafil) is one of unlikely serendipity. Famously developed as a blood pressure treatment, Viagra soon proved to have an unexpected—and highly lucrative—sexual side effect, helping men maintain erections. That’s led to its runaway success since FDA approval on March 27, 1998: Viagra brought in about $1.6 billion in 2016 global sales. It had some of the fastest prescription uptakes and sales growth of any medication, ever, after its launch, pulling in a cool $2 billion in annual sales by 2008. That means it’s well into tens of billions in revenues since the 1998 debut.

Subscribe to Brainstorm Health Daily, our newsletter about the most exciting health innovations.

If that sounds like a little less than what you would have thought (especially given that some 30 million men in the U.S. alone and somewhere around half of those aged 40-to-70 years suffer from a degree of ED), consider that Pfizer had to deploy one of the most successful advertising campaigns in history to notch those sales. (This doesn’t even take into account the competitors it’s spawned, like Eli Lilly’s Cialis and Bayer’s Levitra, while making “erectile dysfunction” a newly common part of the medical lexicon).

All prescription drugs eventually stare down the specter of the dreaded “patent cliff,” when a product loses intellectual property protection. This has been happening in stages for Pfizer (last fall, a version of Viagra was made available over-the-counter in the U.K, for instance). But the company has also taken aggressive steps to guard against revenue loss and protect its little blue baby.

Pfizer announced a generic Viagra version in the U.S. in December that would slash its list price in half, or a little more than $30 per pill. There’s one significant branding difference for that new iteration, though—the pill is white.