Biogen Inc (NASDAQ: BIIB) will miss quarterly sales expectations for its Tecfidera treatment for multiple sclerosis Friday but the disappointment shouldn't roil the company's shares, an analyst said this week.

Morgan Stanley's Matthew Harrison said Biogen in the past has proven volatile after missing Tecfidera expectations.

But Harrison said a promising outlook for Biogen's proposed Alzheimer's treatment BIIB037 "provides a new floor" that could dampen volatility.

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Harrison expects Biogen's overall results will meet Wall Street consensus of $3.91 a share on revenue of $2.66 billion.

But Tecfidera sales of $890 million will miss consensus expectations by about 2 percent, according to his estimate.

Separately, Harrison said Gilead Sciences, Inc. (NASDAQ: GILD)'s Harvoni treatment for hepatitis C doesn't appear threatened by recent drug trial data from Merck & Co., Inc. (NYSE: MRK).

Results from Merck's study of its grazoprevir/elbasvir hepatitis drug "continue to position Harvoni as first-in-class."

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