NEW YORK (CNN/Money) - Bristol-Myers Squibb Co. is the subject of a new Securities and Exchange Commission probe, news that sent the drugmaker's stock plunging Thursday and added to an already long roster of U.S. companies under investigation.

The news was also the latest of several blows to the drugmaker's reputation, coming amidst the controversy surrounding its partner in marketing the anticancer drug Erbitux, ImClone Systems Inc. (IMCL: up $0.31 to $6.83, Research, Estimates) ImClone's CEO, Sam Waksal, has been accused of insider trading, and his friend Martha Stewart is being investigated for her trades of ImClone stock.

Shares of Bristol-Myers (BMY: down $1.04 to $22.11, Research, Estimates) were down more than 7 percent in afternoon trading. At one point in the day, they dipped below $20 per share for the first time since 1996.

Since April, the SEC has been informally investigating the New York-based drugmaker's sales practices last year. The company made so many sales to wholesalers last year that inventories built up too much, causing the company to have to restate its revenue and earnings expectations for 2002.

The SEC wants to know if the company inflated sales purposefully by offering incentives to wholesalers -- including warnings that it planned to raise prices on some of its products -- in order to meet last year's revenue targets. Analysts have said the company's sales practices may have boosted revenue by as much as $1 billion.

The company has denied any wrongdoing and said it was cooperating with the SEC. Company officials said they have been meeting with the SEC, which has made no allegations against it, since April.

Analysts were concerned that Bristol-Myers might end up restating its revenue for 2001. The company has said it doesn't think that will be necessary.

Pharmaceutical companies sometimes boost short-term demand for their products by announcing future price increases. Drug wholesalers will build up inventories of various drugs hoping to avoid the price hike. Such moves increase pharmaceutical company revenue in the short term, but lead to oversupply.

The Financial Times, which broke the story, suggested in a report that the SEC, prompted by recent accounting scandals, may be looking to rein in such pricing practices at Bristol-Myers.

Credit rating agency Standard & Poor's, which put the company on Credit Watch with negative implications when the company slashed its revenue forecasts in April, said Thursday it could cut the drugmaker's rating from AAA, the highest rating available.

The news makes a bad year for Bristol-Myers even worse. Just before the new year, the Food and Drug Administration refused to approve Erbitux, a drug in which Bristol-Myers invested about $2 billion.

Throughout the first months of the year, the drugmaker lost its exclusive rights to make three drugs, including anti-anxiety drug BuSpar.

Its shares plunged after it cut its earnings guidance in April, and CEO Peter Dolan began a restructuring effort that included the resignation of former CFO Fred Schiff.

Around the time of the eruption of the Waksal controversy in June, Bristol-Myers was hit with an antitrust suit concerning its cancer drug Taxol, and 29 state attorneys general have sued it for allegedly trying to block the introduction of generic drugs.

Also in June, the Wall Street Journal reported that the company hired Goldman Sachs to advise it in ways to fix its problems, including putting itself up for sale.