24/7 Wall St. has tracked the short interest in leading biotech and emerging pharmaceutical stocks. After retreating from many of these stocks in the previous two periods, short sellers returned in early March.

Those that saw especially large upswings in short interest include Celgene, which had a setback in expansion of its Revlimid drug in the United Kingdom during the period, Medivation, which offered weak guidance with its earnings report, and Pharmacyclics, which has seen a lot of insider selling this month.

Amgen Inc. (NASDAQ: AMGN) saw a 9.5% increase in early March to around 11.11 million shares, or 1.5% of the total float. That was the highest level of short interest in the past year. It would take more than three days to close out all short positions.

Celgene Corp. (NASDAQ: CELG) saw the biggest gain in short interest among the biotech companies featured here, 33.8% to more than 6.12 million shares. That was 1.5% of Celgene’s float. The short interest has risen in all but one period so far this year. It would take less than two days to cover all short positions.

Dendreon Corp. (NASDAQ: DNDN) saw its short interest inch up to 44.54 million shares from the 44.13 million at the end of February. That ended nine straight periods in which the number of shares sold short had dwindled, and it represents a whopping 29.6% of the float. But the days to cover plunged from about 20 to about seven.

Gilead Sciences Inc. (NASDAQ: GILD) had a 2.2% rise to more than 89.55 million shares short. That was the highest level of short interest in the past year, after rising in 11 of the past 12 periods. It totaled 5.9% of the total float. The days to cover dropped from eight to seven in the period.

Medivation Inc. (NASDAQ: MDVN) saw its short interest increase 15.4% to 4.00 million shares by the middle of the month. That was the greatest number of shares short since last summer, and it totaled 5.4% of the company’s float. It would take about three days to close out all short positions.

Pharmacyclics Inc. (NASDAQ: PCYC) saw a 33.2% jump in its short interest to around 1.97 million shares. That was the greatest number of shares short since last June, and it was 3.3% of the total float. The days to cover is about two.

Other biotech companies that saw an increase in their short interest between the February 28 and March 14 settlement dates include Questcor Pharmaceuticals, Regeneron Pharmaceuticals and Vertex Pharmaceuticals.

Note that the number of shares sold short in Alexion Pharmaceuticals, Biogen Idec, BioMarin Pharmaceutical, Illumina, Incyte and Seattle Genetics shrank somewhat during the period.

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