WASHINGTON — The FDA warned consumers Friday not to use two unapproved Brazilian diet pills because testing has revealed they may contain tranquilizers, antidepressants and stimulants.

The pills are sold over the Web as Emagrece Sim, or the Brazilian Diet Pill, and Herbathin, the Food and Drug Administration said. The pills are billed as all-natural but may contain active ingredients, including controlled substances, normally found only in prescription drugs like Librium and Prozac.

Their use can lead to serious side effects or injury, the FDA said. The active ingredient in Librium, chlordiazepoxide, can be habit-forming.

"There are dangers to consumers who purchase diet pills that contain drugs of unknown origin and quality," said Dr. Steven Galson, director of the FDA's center for drug evaluation and research.

Another ingredient in the pills, Fenproporex, is a stimulant that the body converts to amphetamine.

Users of the diet pills have flunked drug tests after their urine tested positive for amphetamines, said Terry Hall, laboratory director of Toxicology Testing Service Inc. in Miami.

Hall said tests he conducted on the pills, done at the behest of several people who faced losing their jobs after testing positive for amphetamines, revealed the presence of both Librium and an amphetamine-type compound, Hall said. Other samples he subsequently tested appeared to be drug-free.

"I tested a couple other products and as far as I can see, it is complicated enough that one buying a product over the Internet has no way of knowing what product you're going to get," Hall said.

The pills are typically sold in kits, containing two varieties of capsules, with different "levels" to be used at different stages of a weight-loss regimen, the FDA said.

Herbathin Inc. sells a 45-day supply of the pills through its Web site for $185. A call to the company went unanswered on Friday.

However, its website claims the "natural" pills are made from "over 40 different herbs among other high quality ingredients" and that the herbs "come fresh from the Brazilian rain forest, the Pantanal and some are grown in special nurseries."

The FDA said the pills are imported and distributed by two Florida companies, Emagrece Sim Laboratories Inc. of Miami, and EMIEX Corp. of Doral, Fla. No telephone listing for Emagrece Sim Laboratories Inc. could be found.

Eduardo Ramos, president of EMIEX, said his company acted as the agent for the Brazilian manufacturer of the pills. EMIEX successfully imported 2,666 bottles of the pills in three shipments received between August and September 2005. A fourth shipment, 1,638 bottles received in December, remains blocked in his warehouse after the FDA sampled the pills.

"Apparently they came back with pharmaceutical residues and chemicals," Ramos said. The pills will be either returned to Brazil or destroyed, he added.

The company has no further plans to import the pills, said Ramos, who said he'd been told they were "100 percent natural."

"I know it works. They gave me one bottle. I lost like 15 pounds in less than a month. I had no side effects other than dry mouth," said Ramos, who slimmed down to 215 pounds but stopped taking the pills after hearing news reports on the risks.