LOS ANGELES -- It takes time to get a lean body with hard, sculpted muscles, but not everyone has the time. That's why supplements promising to speed up the process can be so appealing.

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Single mom Jennifer Rosenthal thought using supplements was a good idea.

"I was eating properly and I was going to the gym, but just as everybody else, I had additional time constraints," said Rosenthal.

Rosenthal just wanted to spend more time with her son, so when a bodybuilder friend told her a new supplement could cut her workout time, she was eager to try it.

"He told me basically it took the place of cardio. It put your body in an aerobic state, so you burn additional fat that way. So I thought -- great," said Rosenthal.

The supplement is called usnic acid. Rosenthal started taking the supplement last October. The label recommended taking eight capsules a day. Rosenthal said she took half that amount.

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"For the first couple of days, I was pretty warm, like I was doing like an aerobic workout," said Rosenthal.

Rosenthal said she had no other side effects and lost six pounds -- but then things changed.

"I had symptoms like hepatitis," said Rosenthal.

Rosenthal said her eyes were turning yellow and she was extremely tired and weak. But every test for hepatitis was negative. Her doctors didn't know about the supplement at that point, so they didn't connect it to her symptoms.

Rosenthal kept getting sicker until on Nov. 5, she ended up at Long Beach Memorial Hospital, drifting in and out of consciousness. Two days later, Rosenthal was air-lifted to the University of California-Los Angeles.

"She was critically ill. Her life expectancy was measured in the order of days. The only thing that would save her life was a liver transplantation," said Dr. Sammy Saab, admitting physician at UCLA.

Dr. Francisco Durazo, Rosenthal's doctor, believes usnic acid may have caused her illness.

"I think it's very likely and highly probable (that) this substance is toxic to the liver," said Durazo.

Rosenthal got lucky -- a donor liver became available. But Rosenthal wasn't the only person with liver failure linked to usnic acid.

At Cedars-Sinai Medical Center in Los Angeles, doctors found five other people with liver problems. They called the Food and Drug Administration and discovered two more and published a paper on all seven potentially poisoned by the supplement.

"The one thing they had in common was that they were taking this over-the-counter herb product, and this over-the-counter herb product contained this usnic acid," said Dr. Glenn Braunstein, of Cedars-Sinai Medical Center.

That over-the-counter product was LipoKinetix. After getting several reports linking it to "serious liver injuries," the FDA warned consumers to stop using it. The manufacturer, Syntrax Innovations, said it stopped selling LipoKinetix because of supply issues eight months before the FDA warning. Syntrax still maintains its product was safe.

Rosenthal did not use LipoKinetix; she used a pure form of usnic acid. The man who made it says he stopped selling it as soon as Jennifer's doctors contacted him.

Rosenthal is now getting her life back on track.

"I'm healthy. I'm back to doing a lot of the things I did before my transplant. But I still have some limitations, which, those will be the same limitations that I'll have forever," said Rosenthal.

Just five months after her transplant, Rosenthal ran a 5K race to help raise awareness of the need for organ donors. She's also hoping to raise a red flag for anyone still taking usnic acid.

"I think it's my duty to say, 'Here's the potential danger,'" said Rosenthal.

Several products containing usnic acid have been taken off the market, but some are still available. Read the labels and check with your doctor before taking any supplements.

Doctors don't know why some people are affected by usnic acid and others are not; they say the supplement still needs more study.

If you have a problem with any supplement, report it to the FDA immediately. Although the FDA doesn't regulate the supplement industry, it has a program called MedWatch for reporting and tracking problems with any health-related products.