WASHINGTON (CNN) -- A consumer watchdog group says a plan to allow drugs to be promoted as treatment for diseases other than the ones for which they are approved is bad news for patients.

Doctors often discover that a drug approved to treat one ailment can effectively treat another. For example, the celebrated impotency drug Viagra was originally approved to treat chest pain caused by heart disease.

It is already legal for doctors to prescribe drugs for unapproved purposes, called "off-label" uses. However, until now, drug companies could not officially inform physicians, pharmacists or insurers of other potential uses.

A new law changes that, and the U.S. Food and Drug Administration is proposing new rules that will implement the law.

The consumer watchdog group Public Citizen is critical of the plan.

"In a sense, huge numbers of people are going to be made guinea pigs for unapproved uses of drugs," said Dr. Sidney Wolfe of Public Citizen.( 179K/13 sec. AIFF or WAV sound)

Viagra was originally used to treat chest pain

The FDA's new rules would allow the companies to promote off-label uses while placing restrictions on the kind of information distributed in the process. For example, the regulations would prevent companies from distributing false or misleading information.

Drug companies say the requirements are strict enough, and that the new law makes sense.

"There are many patients who use drugs off-label that if they had to wait for it to come on-label would be tremendously disadvantaged," said Dr. John Siegfried, senior clinical advisor for Pharmaceutical Research and Manufacturers of America.( 204K/16 sec. AIFF or WAV sound)