Thousands of over-the-counter cold medicines could be much more potent than intended, according to a discovery by an Australian pharmaceutical company.
This means people could suffer side effects such as nervousness and increased blood pressure, says AFT Pharmaceuticals.
The New England Journal of Medicine has published a letter about its findings.
At issue is the nasal decongestant phenylephrine, which is widely used in over-the-counter medicines as a replacement for pseudoephedrine.
In many countries pseudoephedrine-based products are available only on prescription because of its use in making the drug methamphetamine.
The maximum allowed dose of phenylephrine in over-the-counter products is 10mg. But combining it with paracetamol boosts it to the equivalent of 20mg, according to the research.
"People have inadvertently been taking the equivalent of double the recommended levels of phenylephrine," said AFT managing director Dr Hartley Atkinson.
"We immediately informed Medsafe in New Zealand and the Therapeutic Goods Administration (TGA) of our findings."
However, pharmacists say there have been no concerning safety signals.
University of Sydney Professor Andrew McLachlan said the discovery was interesting, but the drugs had been widely used in many products for several years without raising safety concerns.
"There are systems in place in Australia and New Zealand to identify issues."
A TGA spokesperson said it was considering what action, if any, was needed.
