Natural cold and flu busters unlikely to do the trick but could do harm

But a top pharmacist says there is little evidence these so-called natural remedies actually work.

And some common ingredients, such as echinacea and turmeric, can cause more harm than good.

If people insist on taking them, it is important they consult a pharmacist to check the supplements they are taking don't interact with other drugs, warns top pharmacist Geraldine Moses in an editorial published in Australian Prescriber on Monday.

"Every step of how drugs get around your body, complementary medicines can interfere. They can be real troublemakers," Associate Professor Moses said.

Professor Moses works as a clinical pharmacist at Mater Health Services in Brisbane, and holds a position with the University of Queensland's school of pharmacy.

She has seen several people hit by side effects when they mix prescription medicines with complementary remedies.

Echinacea is a herb commonly marketed to shorten the length of a cold. But it can get people into real trouble, she says.

The herb can worsen autoimmune conditions like asthma, making any medicines a person might be using less effective.

Professor Moses has seen people with asthma, as well as rheumatoid arthritis and multiple sclerosis, who have had their symptoms worsen after taking echinacea.

Turmeric, another popular supplement - "it's in bloody tea and coffee now," says Professor Moses - is also a blood thinner.

High doses of turmeric combined with blood-thinning medication can increase the risk of internal bleeding, and pose risks before and after surgery.

"We're not worried about the turmeric in a latte or a foul-tasting coffee," Professor Moses says. "But the tablets and capsules [people] buy, they are not natural any more, they have been made in a factory. And that's where we worry."

Professor Moses also lists black pepper, schisandra fruit, goldenseal root, green tea, guarana and yerbe mate as known to alter the way conventional medicines work. But there are many others.

Of the common alternative remedies, she nominates zinc as one that might actually work. "I believe in that one."

In studies, zinc appears to cut cold length by a couple of days - but with significant side effects.

Zinc nasal sprays are most effective but can cause people to lose their sense of smell, in some cases permanently. Zinc lozenges can leave a metallic taste in the mouth for weeks.

Professor John Dwyer, a former director of medicine at the Prince of Wales Hospital in Sydney, has this advice for people considering natural remedies: don't.

"There have been studies done on everything, and there is no consistent evidence that any of it makes a difference. There is no consistent evidence that anything works. And there is always a risk."

Many natural and complementary medicines can be bought at pharmacies. That lends them an air of legitimacy, Professor Dwyer says.

But often the pharmacist at the back of the shop has nothing to do with the pills and potions on the shelves elsewhere.

"For many pharmacists, profit has replaced professionalism. The number of pharmacies advocating remedies for which there is basically no evidence is really quite distressing, given these are men and women of science," he says.

A spokesman for the Pharmacy Guild of Australia said it was up to individual pharmacy owners as to whether they stocked complementary medicines.

"Pharmacists, as health professionals, have a duty of care to be aware of available clinical evidence that supports the therapeutic and marketing claims made about all products sold in their pharmacies."

Professor Dwyer's best advice for people with a cold? "Stay home. They will get over it faster if they rest."