New Zealand's consumer watchdog has found most cold and flu medicines are a waste of money and no better than paracetamol, honey and rest.

Consumer NZ found many products, some costing up to $30, had unconvincing scientific evidence to back up their claims and were little more than placebo effects. Kiwis forked out $38million for "pills and potions for fighting winter bugs", Consumer NZ reported.

Medsafe have been asked to investigate.

"People hit by a winter bug may want to save their money and stick with bed rest and a painkiller," Consumer NZ chief executive Sue Chetwin said.

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"Consumers who buy a product that says it will clear their nose and ease their cough should be able to expect there's consistent evidence to support the claim. But that's not the case with many cold and flu products."

Consumer NZ looked at more than 50 cold and flu products, from decongestants and antihistamines through to cough drops, to see which offer the most effective relief.

It found more convincing evidence that staples such as paracetamol, ibuprofen and aspirin can help ease the aches and discomfort from cold or flu. Studies also suggest decongestant nasal sprays may help ease that "bunged-up" feeling.

The report reviewed research on typical ingredients in cold and flu remedies.

Throat lozenges were found to be no more effective than sucking a lolly.

Chetwin said independent systematic reviews of the ingredients found in common cough medications (dextromethorphan and guaifenesin) have not found good evidence of their effectiveness.

Much of the supporting research on the decongestant phenylephrine was also conducted more than 40 years ago and many of the studies had design flaws, she said.

Patients could easily double-dose as the same ingredients are in multiple medications.

Consumer NZ had raised its concerns with Medsafe, the Ministry of Health agency tasked with ensuring over-the-counter medication is safe and effective.

"We have asked Medsafe to review the effectiveness and marketing of these ingredients. Like any product, cold and flu remedies should be true to label and do what they say they'll do", Chetwin said

University of Auckland medical professor Bruce Arroll told Consumer NZ the best thing you can ask your doctor for is a sick note to take a few days off work, as this stops you passing on the sickness.