Shares of the pharmaceutical giant Pfizer (PFE) - Get Report were lower early Thursday morning following a downgrade of the stock at Credit Suisse over expiring patents for two important drugs.

The firm lowered its rating on Pfizer to "Neutral" from "Outperform" as the pharma giant is faced with two "meaningful" expiring patents for its Viagra and Lyrica male impotency treatment drugs.

"That will pose a further headwind to both top and bottom-line growth," Credit Suisse analyst Vamil Divan wrote. "We have been long-time supporters of the PFE story and see opportunities for upside over the long-term, but see more limited drivers of upside over the next 6-12 months."

Divan decreased his 2018 Pfizer earnings per share forecast to $2.71 from $2.78, projecting that the company will lose its U.S. patent exclusively for Viagra in December and for Lyrica in 2018.

Those two drugs alone accounted for more than $4.2 billion in domestic sales last year, he added.

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