SOUTH FLORIDA -- Some doctors warn that some popular diet pills could be dangerous.

Over-the-counter diet pills known as the "Brazilian pills," claim to be all-natural, but lab tests have found that some variations of them contain illegal amphetamines, NBC 6's Jaylene Rodriguez reported.

At least four unsuspecting women taking the diet pills failed pre-employment drug tests at Baptist Health South Florida, Rodriguez reported.

Thomas Moore, the corporate director of employment health services at Baptist Health South Florida, said the women were very surprised.

Moore said he realized there was a common thread among the women when the third case happened in a month. He sent the pills the women were taking to a lab to be analyzed.

"In most of them, it was amphetamines. We found a drug called diazepam, which is a Valium," Moore said.

The pills could be life-threatening, Rodriguez reported.

A spokesperson for Kendall Regional Medical Center confirmed that two women who were admitted into the hospital's critical care unit with dangerous blood sugar levels were both taking Brazilian pills.

Doctors have not confirmed the pills caused the women's health problems, but suspect they might have. The women survived.

Despite the potential dangers, the pills can still be purchased on the Internet for up to $200 for a month's supply.

Carolina Siervo discovered the pills, which she calls a "miracle" weight loss system. She said she lost 60 pounds in four months.

She is happy with her results so far, and has no plans to stop taking the pills.

"If it's too good to be true, they are probably the wrong thing to take," Moore said.

The pills could elevate your blood pressure to a dangerous level, and there is a possibility that some patients could become addicted, experts said.

There are some variations of the "Brazilian" pills that are not considered dangerous, but doctors warned that based on the label, it is too difficult to tell which ones are dangerous and which ones are not.