What happened

Allergan plc (NYSE:AGN) stock had another dismal month after Mylan (NASDAQ:MYL) took aim at two of the company's largest revenue streams: Restasis and Botox. Shares of the drugmaker slid 14.4% in February, according to data from S&P Global Market Intelligence.

So what 

Combined Botox and Restasis sales totaled $995.9 million in the fourth quarter, which works out to around 23% of total revenue during the period. Last fall, Allergan transferred patents related to Restasis to the St. Regis Mohawk Tribe to thwart Mylan's efforts to launch a generic version of the popular dry-eye treatment. Late last month, the U.S. Patent Trial and Appeal Board denied the tribe's motion to terminate the challenge. Cheap generic competition could decimate sales of branded Restasis, which carries a wholesale acquisition cost that works out to around $5,000 per year.

Frustrated person in front of computer monitors and keyboard.

Image source: Getty Images.

To end February, Mylan fired a second shot across Allergan's bow in the form of a collaboration and license agreement with Revance Therapeutics (NASDAQ:RVNC). The partners intend to bring a biosimilar version of Botox to major markets around the globe.

Now what

Revance's proprietary botulism toxin, RT002, disturbed Allergan investors when it entered late-stage clinical testing last December. A Botox biosimilar from the two partners could take years to develop, but proceeds from the partnership could help Revance speed up RT002's development.

While industry peers take aim at the Allergan's most popular products, a drug investors had hoped to offset the losses may have hit a big snag. Allergan has forecast blockbuster sales for Esmya, but a recent safety scare will dampen enthusiasm for its rollout. European investigators recently began investigating safety issues related to Esmya after regulators noticed four cases of serious liver damage.

Cory Renauer has no position in any of the stocks mentioned. The Motley Fool recommends Mylan. The Motley Fool has a disclosure policy.