NSW's health authority is warning the public about illegal weight loss drugs known as 'shredders', after numerous deaths in Australia and overseas.

Details on the deaths cannot be made public as they are under police investigation, but NSW Health says products containing the chemical 2,4-dinitrophenol (DNP) can cause death within hours.

The drugs are illegal, with the sale, use and supply of the substance prohibited.

Marianne Gale from NSW Health said DNP prevents energy being stored as fat in the body, releasing it as heat instead.

Dr Gale said that raises body temperature, causing serious health issues.

"It can damage the cells of organs, such as muscles, kidneys, and the brain," Dr Gale said.

"People can become seriously unwell within hours of ingestion.

"This can include profuse sweating. People can experience a fast heart rate, fast breathing, and often they can progress quite quickly to having a seizure, to going into a coma and even into death."

Dr Gale said there was no antidote, and people had died after using these products even with the best medical care.

"There's been reports of deaths internationally over recent years for quite some years, and we're aware that more recently in Australia that there have been several deaths across the country," she said.

Dr Gale said shredders are often marketed to people in the weight loss and bodybuilding communities.

"People should be extremely cautious of purchasing any substances online, and particularly avoid any products that name this chemical DNP or any product from an unverified source that might be marketed as a weight-loss agent," she said.