Many popular over-the-counter cold and flu remedies risk being pulled from shelves in the US after its drug regulator ruled a key ingredient to be ineffective.
On Thursday, the Food and Drug Administration (FDA) said oral phenylephrine is not an effective nasal decongestant.
Phenylephrine is the main decongestant ingredient in soluble and tablet-form cold-and-flu medicines such as Lemsip, Beechams, and Sudafed.
It's meant to work by reducing the swelling of blood vessels in the nasal passages.
Other ingredients, such as paracetamol, help bring down temperatures, and relieve aches and pains.
The FDA said it had come to its conclusion after an "extensive review of available data".
The ruling does not affect medicines available in the UK, with the Medicines and Healthcare Products Regulatory Agency's (MHRA) chief safety officer Dr Alison Cave saying there are "no safety concerns" over phenylephrine products and "people can continue to use as directed".
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A group of advisers to the FDA concluded it was ineffective a year ago after research from the University of Florida showed phenylephrine products were no more effective than placebo pills at relieving symptoms.
Dr Patrizia Cavazzoni, director of the FDA Centre for Drug Evaluation and Research, said: "We are taking this next step in the process to propose removing oral phenylephrine because it is not effective as a nasal decongestant."
The decision is not final, it added, which means US retailers can still stock medicines that contain the drug.
But if it is made final, American stores and websites would be banned from selling medicines containing phenylephrine and pharmaceutical companies would be forced to produce nasal decongestants instead - or find a different ingredient for oral remedies.
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The US industry body, the Consumer Healthcare Products Association, has said banning oral phenylephrine would have a negative impact on consumers.
Michelle Riddalls, chief executive of the UK's Consumer Healthcare Association, who represents UK drug manufacturers, told Sky News: "We understand that consumers might be concerned or confused about the news on phenylephrine (PE) and what this might mean for them and their families, especially at this time of year, with cough, cold and flu season upon us.
"Consumer safety is paramount to our members, including those who manufacture products containing phenylephrine. We want to reassure consumers that FDA's consultation is not relating to a safety issue.
"Nasal decongestants containing PE have been trusted by health professionals and consumers for decades and are still available to help treat symptoms of common respiratory infections, such as cough, cold and flu."
Sky News has contacted Lemsip, Beechams, and Sudafed for comment.