Biogen is crashing after cutting the outlook for its blockbuster drug

Biogen shares fell more than 16% in early trading on Friday after quarterly earnings results that beat on profits but missed on revenues.

Also, the biotech giant lowered its forecast for full-year earnings and sales because it doesn't expect Tecfidera, its key multiple sclerosis drug, to grow as expected.

Revenues from the drug rose 26% to $883 million year-over-year in Q2, slower than the 63% rate recorded in the previous quarter, according to the Wall Street Journal.

The company reported revenues of $2.6 billion, missing the consensus forecast for $2.73 billion according to Bloomberg.

In the earnings statement, CEO George Scangos said, "TECFIDERA, which is now the most prescribed oral MS therapy globally, is experiencing moderated patient growth following rapid initial uptake."

Adjusted earnings per share came in at $4.22, ahead of estimates for $4.1.

Biogen shares are down 6% year-to-date and 4% over the past 12 months. Today's dive took the stock to the lowest levels since early December.

On Friday morning, Piper Jaffray lowered its price target on the stock to $361 from $410 after a "lackluster" quarter, according to The Fly On The Wall.

The iShares Biotechnology Index fell 2% on Friday morning, but it's up 27% year-to-date and 53% over the past year, with opinions on Wall Street mixed about whether the sector is in a bubble.

Here's a chart showing the drop in Biogen shares on Friday morning: