Short sellers are retreating from earlier gambles on biotech stocks.

Companies from Gilead Sciences (GILD) to Onyx Pharmaceuticals (ONXX) saw less interest from short sellers in August. A few notable exceptions were Amgen (AMGN), for which short interest rose by 8.8% to 12.8 million shares by Aug. 31; Arena Pharmaceuticals (ARNA), which saw short interest increase 2.7% to 43.9 million shares; and Illumina (ILMN), which saw short interest rise 7.2% to 23.6 million.

Here is a closer look at the biotech stocks which saw the largest declines in short interest between Aug. 15 and Aug. 31.

Biogen Idec (BIIB)

This S&P 500 component saw short interest fall 5.6% to 2.1 million shares. Biogen Idec develops and markets therapies for the treatment of neurodegenerative diseases, hemophilia and autoimmune disorders. It has a market capitalization near $36 billion and a return on equity of nearly 22%. Biogen Idec set a deal with Regulus Therapeutics in late August identify biomarkers for multiple sclerosis. Shares are up about 34% this year and trading near a multiyear high.

Celgene (CELG)

Short interest in this maker of therapies to treat cancer and immune-inflammatory related diseases declined 8.4% to 5.8 million shares. The return on equity of this S&P 500 component is a healthy 26%, and the long-term earnings per share (EPS) growth forecast is more than 22%. The company said in late August that it may increase its stake in Globelmmune. About 23 out of 28 analysts polled by Thomson/First Call recommend buying shares of Celgene; none recommend selling.

Gilead Sciences (GILD)

Shares sold short in Gilead Sciences dropped 10.4% to 18.8 million. This maker of therapeutics for the treatment of life-threatening diseases such as HIV and hepatitis has a market cap near $45 billion. It too is an S&P 500 component and has a return on equity is more than 36%. The company received FDA approval of hits HIV drug Stribild at the end of August. Out of 29 analysts surveyed, 22 rate the stock a "buy" or "strong buy;" none recommend selling shares.

Onyx Pharmaceuticals (ONXX)

Short interest in Onyx Pharmaceuticals fell 6.7% to 5.35 million shares. This South San Francisco company develops therapies that target the molecular mechanisms that cause cancer. Its long-term EPS growth forecast is more than 19%, and the mean price target is about 18% higher than the current share price. Its market cap is near $5 billion. Back in August, Onyx filed for FDA approval of its drug regorafenib as a treatment for certain gastrointestinal tumors.

Vivus (VVUS)

This Mountain View, Calif. biopharmaceutical company saw short interest fall 4.2% to 15.7 million shares. That is about 15% of the float. The market cap is more than $2 billion. The long-term EPS growth forecast is about 51%. The mean price target of analysts is about 40% higher than the current share price and would be a multiyear high. But shares have traded mostly between $21 and $23 since July, though they are up more than 126% this year.

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