WASHINGTON (Reuters) - Ephedra makers came under attack in Congress on Wednesday with a prominent lawmaker calling their conduct unacceptable and threatening tougher regulation of the dietary supplement, which has been linked to side effects that include heart attack and stroke.

"If they don't clean up their act, I can promise you we will do it for them," Rep. Billy Tauzin, a Louisiana Republican, told a House Energy and Commerce subcommittee hearing. Tauzin chairs the full committee.

"We've learned that these ephedra supplement makers have been engaged in some highly questionable behavior, from producing products without any safety testing to promoting safety and efficacy based on dubious industry-sponsored studies," Tauzin said.

Government officials have warned consumers not to take the stimulant and proposed a warning label for ephedra products that mentions heart attack and death as possible side effects.

Consumer groups say ephedra is too dangerous and have called for a ban.

Ephedra makers insist the product is safe when taken as directed; but one manufacturer, privately-held Metabolife International, said in a statement that it supports the proposed warning label.

"Supplements containing ephedra are not for everyone," the company said. It noted that its product, Metabolife 356, comes with a warning that advises people with health problems such as heart disease to consult a doctor before use.

Metabolife said ephedra has helped many people battle obesity, a major U.S. health problem.

"Millions of consumers throughout the United States use ephedra dietary supplements as safe, inexpensive and effective means by which to support weight loss," the company said.

"To our knowledge, there is insufficient evidence to demonstrate that ephedra supplements are unsafe when taken as directed," the firm added.

The investigations subcommittee subpoenaed two former Metabolife executives and one current employee after they declined to appear voluntarily at the hearing.

Rep. Jim Greenwood, a Pennsylvania Republican and chairman of the investigations panel, said the three witnesses were expected to assert their constitutional right against self-incrimination and not testify.

But Metabolife Chief Executive Russell Schreck was scheduled to testify voluntarily.

A RAND Corp. study released by U.S. health officials in February said ephedra was associated with higher risks of mild to moderate side effects such as heart palpitations, tremor and insomnia, especially when taken with other stimulants.

The review of 16,000 adverse events also found two deaths, four heart attacks and nine strokes involving ephedra in which no other contributing factors were identified.

The National Football League, the International Olympic Committee and the National Collegiate Athletic Association have banned ephedra use among their athletes.