In updated guidelines for consumers, the FDA warns that homeopathic pills "have the potential to cause significant and even permanent harm if they are poorly manufactured."
The agency points out that the remedies are "made from a wide range of substances, including ingredients derived from plants, healthy or diseased animal or human sources, minerals and chemicals, including known poisons."
The FDA has ramped up its warnings after consumer interest in homeopathy has been on the rise. Recent surveys suggest up a third of adults are using homeopathic remedies in the US, and by 10 per cent of people in the UK.
It has a special warning for children and the elderly, whose immune systems may be too weak to protect them from homeopathic substances.
The FDA recently issued warning letters to 10 manufacturers of homeopathic eye drops, claiming the packaging was not sterile.
Homeopathy is based on the idea that water has memory, and the greater the dilution the greater the potency—but its critics argue this is nonsense and goes against known scientific laws. The original ingredient is diluted many hundreds of times and so nothing of it can still be in the actual pill.