The glory days for Pfizer's ($PFE) Viagra are winding down in the U.S., and now, another generics company has the go-ahead to help pave the way for the drug's demise.

Under a settlement agreement, Mylan ($MYL) is free to launch its copy of the little blue pill as early as Dec.11, 2017--or sooner, under certain conditions, the company said Monday. With that pact, it'll be able to start attacking the blockbuster seller well before its 2020 patent expiration.

And Mylan won't be the only company jumping into the fray. Last December, Teva ($TEVA) inked its own pact to launch a knockoff on Dec. 11, 2017--meaning the two generics heavyweights are in for some serious price competition from the get-go.

That will likely mean a bigger drop in branded sales for Pfizer, which has already gotten a taste of that in ex-U.S. markets. Since 2012, revenue from the pill in the EU, Japan and Australia has shrunk from $472 million to $146 million as the patents there expired.

That's partly because Viagra-copiers came out swinging in those other markets, using price as a selling point. In South Korea, Hanmi Pharmaceuticals advertised its 5,000 won ($4.25) cost per 100-mg tablet--a third of the cost of the branded med--to win an advantage over the 5 other companies that launched generics within two weeks of Pfizer's patent expiration there. And when Pfizer's IP shield fell in the EU nearly two years ago, industry watchers expected U.K. prices to drop from about £10 a pill to about £1.

If historical patent expirations are any guide, Pfizer will face a tougher battle to hang onto Viagra branded sales with two generics makers in the mix. And in the U.S., Viagra has farther to fall than it did in other countries. Last year, the erectile dysfunction therapy generated $1.3 billion in Pfizer's home country, according to IMS Health estimates.

- read Mylan's release