The Food and Drug Administration told eight companies Thursday to stop marketing a series of untested drugs and devices as treatments for autism and a slew of other conditions, citing serious side effects including death.

At issue are chelation products sold without a prescription, which the FDA says it has not evaluated and are unproven. In recent years, such treatments have proliferated through online sales, often marketed as dietary supplements.

The companies cited this week claim their products can treat everything from autism to Parkinson’s disease, heart troubles and macular degeneration.

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“These products are dangerously misleading because they are targeted to patients with serious conditions and limited treatment options,” said Deborah Autor, director of the office of compliance at the FDA’s Center for Drug Evaluation and Research.

Chelation is used to remove heavy metals from the body. The FDA does approve the treatment in some cases, but only by prescription and for conditions like lead poisoning or iron overload, as it can cause side effects including dehydration, kidney failure and death, the agency says.

Warning letters were sent to World Health Products, Hormonal Health, Evenbetternow, Maxam Nutraceutics, Cardio Renew, Artery Health Institute, Longevity Plus and Dr. Rhonda Henry.

The companies have 15 working days to respond to the FDA. If the problems are not corrected, they face the threat of legal action.