Pfizer Inc.’s (NYSE: PFE) patent protecting popular erectile dysfunction drug Viagra from generic competition may not expire until April 2020, but Teva Pharmaceutical Industries Ltd. (NYSE: TEVA) has acquired rights to launch its generic version in the United States in December of 2017, or perhaps sooner.

Viagra is one of Pfizer’s best-selling drugs, generating about $1.2 billion in sales in the United States last year and more than $2 billion worldwide. Pfizer announced Tuesday that Teva Pharmaceuticals USA will pay an undisclosed royalty for a license to produce its copycat version of Viagra. The terms of the settlement agreement were largely confidential.

The two companies have come to terms after Pfizer sued Teva back in 2010 over its plans to introduce a generic version of Viagra. Teva argued that Pfizer’s U.S. patent on Viagra is invalid. The deal announced Tuesday is part of the settlement of the patent suit.

Viagra has the same active ingredient as Pfizer’s heart medication Revatio — sildenafil citrate. The patent on the active ingredient had already expired, but Pfizer received an additional patent on the use of the drug to treat erectile dysfunction, extending its exclusivity until April 2020. The validity of Pfizer’s patent was upheld by a federal judge in 2011.

In addition, Eli Lilly and Co.’s (NYSE: LLY) erectile dysfunction drug Cialis loses its patent protection in 2017. Drugmakers fight aggressively to protect patents because sales of their drugs typically plunge after cheaper, generic versions hit the market.

Shares of Pfizer and Teva were inactive in premarket trading Wednesday. The former closed at $30.14 Tuesday, in a 52-week range of $24.63 to $32.50. The latter closed at $39.84 in a 52-week range of $36.26 to $41.74.

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