NEW YORK (CNN/Money) - Analysts speculate that Bristol-Myers Squibb may have become a takeover target after its shares fell to a four-year low following disappointing results from a hypertension drug, according to published reports Thursday.

A study of Bristol-Myers' Vanlev medication for high blood pressure and heart failure found that the drug did not significantly improve patient conditions compared with enalapril, a generic form of Merck & Co.'s (MRK: Research, Estimates) Vasotec.

Vanlev also induced more cases of angiodema, a potentially deadly side-effect that causes swelling in the face and neck area, than enalapril.

Bristol-Myers (BMY: Research, Estimates) shares tumbled $7.57, or 15.5 percent, Wednesday to close at $41.08 on the New York Stock Exchange.

The Vanlev results come on the heels of another disappointment for New York-based Bristol-Myers when the U.S. Food and Drug Administration withheld approval of Erbitux, a drug developed by the company in tandem with ImClone Systems.

"We're left with a company with a litany of problems and a management that has zero credibility," Barbara Ryan, an analyst with Deutsche Banc Alex. Brown, told the Wall Street Journal. "I think some heads will roll and they will be very vulnerable to take-out."

Ryan suggested that Basel, Switzerland-based Novartis AG or London-based AstraZeneca (AZN: Research, Estimates) may be interested in acquiring Bristol-Myers, according to the New York Times.

Last year, Bristol paid $7.8 billion for DuPont's (DD: Research, Estimates) pharmaceutical operation, which does contain a potentially effective medication for rheumatoid arthritis, "but we won't know how effective for a year or two," said Richard Evans, an analysts with Sanford C. Bernstein, according to the Times.

"They either need a major new product or they need a merger," Evans said.