WASHINGTON - Advisers to the Food and Drug Administration said Friday a blood-clotting drug from GlaxoSmithKline PLC and Ligand Pharmaceuticals Inc. is effective for short-term use, despite reservations by FDA scientists.

A panel of 16 outside advisers voted unanimously in favor of Promacta at a meeting held in Chicago, said FDA spokeswoman Karen Riley. The drug is designed to treat an immune system disorder that causes the body to destroy its own platelets, which can cause excessive bleeding.

The decision sent shares of San Diego-based Ligand soaring $1.16, or 49.4 percent, to $3.51 on heavy volume. Shares of GlaxoSmithKline PLC rose 55 cents, or 1.3 percent, to $44.53.

Panel members noted that the companies have not provided long-term data on the drug, but said results from two, six-week studies appeared to show significant benefit. Long-term studies should be available by the end of the year, according to the companies.

British drugmaker Glaxo and partner Ligand want Promacta approved to treat patients’ with chronic idiopathic thrombocytopenic purpura, which causes excessive bruising and bleeding. About 60,000 people in the U.S. have the disease, according to Glaxo.

The FDA has until June 19 to make a final decision on the drug. It is not required to follow the advice of its outside panel, though it often does.

The panel’s opinion came as a surprise to Wall Street, since the FDA on Wednesday released its own negative review of the drug. That news walloped Ligand’s shares, sending them to their lowest point in at least 10 years, $2.16, only to recover by day’s end at $2.30, still down 36 percent. The company has been a Glaxo partner since 1997.

Agency reviewers said that the drug did not significantly lower bleeding compared with a placebo in two studies submitted by the companies.

FDA scientists also raised concerns about liver toxicity and the drug’s safety in long-term use. Although the companies are seeking approval for short-term use, the FDA cautioned that doctors could continue prescribing the drug for extended periods of time.

Susquehanna Financial Group analyst Derek Jellinek faulted GlaxoSmithKline’s clinical testing strategy for the critical FDA documents. While the company applied for short-term use for Promacta, GlaxoSmithKline instead studied patients with the chronic condition who need long-term treatment, he said in a note to investors Thursday.

That choice of chronic patients could complicate the company’s dealings with the FDA, he said, “because the agency believes that short-term data may not immediately translate into long-term results.”

Jellinek reaffirmed a “Positive” rating but lowered his price target to $7 from $8.

Friday’s panel vote could put pressure on biotech giant Amgen, which is awaiting an approval decision on a similar drug called Nplate. The same FDA advisory panel that supported Promacta on Friday also unanimously supported Amgen’s drug in March. The FDA is scheduled to make a decision on Nplate by July 23.

Amgen is seeking approval for long-term use of its drug, a company spokeswoman noted Friday, which would distinguish it from Promacta. Amgen also has conducted three years worth of studies on patients with the blood disorder, the longest of any company, she said.

Shares of Thousand Oaks, Calif.-based Amgen Inc. rose 61 cents, or 1.4 percent, to $44.03.