Harmful pharmaceuticals - including one withdrawn almost 20 years ago over a cancer risk - have been found in weight loss products bought from overseas websites and sent to Australia.

The Therapeutic Goods Administration is warning of the dangers of buying weight loss medicines online, particularly from unknown overseas websites.

Many sites offering "herbal" and "drug-free" products in fact had hidden ingredients that could be harmful, TGA's principal medical adviser Tim Greenaway said on Tuesday.

"Not only have many of these products not been evaluated for safety by the TGA, some are not approved for supply in Australia," he said.

Dr Greenaway said it was most concerning that analysis of seized products by the TGA laboratories found products that contain contaminants including pharmaceutical substances not listed on labels.

The substances found include phenolphthalein, which was withdrawn from sale in many markets in the late 1990s due to concerns over an increased risk of cancer with long term use.

The TGA also found some products had sibutramine - a prescription-only substance in Australia that was withdrawn from world markets in 2010 due to increased risk of cardiac events like heart attacks and stroke.

Dr Greenaway urged people to use caution when buying weight-loss medicine online and to contact their pharmacist or GP if they have concerns about products they have taken.