ENERGISED: Sharyn McLean admits she was prepared to have faith that her magnetic bracelet would improve her fatigue, and is pleased with the results.

Colloidal silver, deer velvet, arnica and rescue remedy are a "waste of time and money" and sometimes harmful, doctors say.

In the latest New Zealand Medical Journal Digest, doctors Shaun Holt and Sarah Jeffries and health psychologist Andrew Gilbey have slammed some of New Zealanders' favourite natural health products as ineffective.

Holt told The Press that of the "hundreds" of therapies and products, about 95 per cent were either not biologically plausible or not supported by research evidence.

Popular but ineffective products and therapies included deer velvet, rescue remedy, arnica, propolis, magnets, shark cartilage, the lemon detox diet, and megadoses of vitamin C to treat cancer.

Some products, such as colloidal silver, which is marketed as being beneficial for the immune system and in fighting diseases such as cancer, HIV and pneumonia, could be dangerous, he said.

"Silver does have some anti-microbial actions, but not only is there no clinical evidence of an efficacy for these serious indications, products have been shown to contain widely variable amounts of silver and can cause argyria - dangerous and untreatable silver poisoning,'' he said.

"The difficulty for people is, which are the 5 per cent of products and therapies worth trying? Use the ones with evidence behind them.

"The problem is people go on the internet, which is not reliable."

People often assumed a product or therapy worked because reputable people endorsed it in advertisements, he said.

"There is no reason deer velvet would work for anything, though it might produce a placebo effect," Holt said.

"It's quite shocking how little research there is, and it's a reasonably big industry."

Gilbey said people were paying big money for products and therapies that did nothing.

"I think people would be quite surprised. It would be lovely if you could get something out of the garden, or scrape something off a tree, and it will fight off cancer, but there are not many of those things around," he said.

Christchurch woman Josie McNeil, of Naturia Health 2000, said she believed natural products and therapies worked on humans and animals.

"I have used colloidal silver on my cat. He was a fighter and he would get big abscesses," she said.

"In one case, it went right through his eye, and I sorted it out with silver. I know it works.

"Do animals get a placebo effect? No. This is where [the doctors] come unstuck.

"Anyone who gets conjunctivitis; if you put drops in your eye, it goes away."

She said two customers used rescue remedy on goats that got stressed when milking, and it calmed them.

Deer velvet was a popular remedy to treat arthritis in animals, she said.

"The thing about herbal remedies is they often take slightly longer than a concoction from a doctor, but natural remedies don't have a detrimental effect, which drugs often do,'' she said.

"I just find they are more gentle, and they are not an alternative medicine - it's the original.

"Since the beginning of time, herbs were given to people for their health and wellbeing, so it's not something new."

Natural-product user Sharyn McLean said she paid $100 for a magnetic bracelet that she had been wearing for about a year.

"I was really, really tired and it's supposed to help with fatigue. I did feel after a period of time a bit more energised,'' she said.

"I don't know whether that was psychological but, to be honest, I don't really care because I felt better. I was prepared to have faith in it."