A very uncommon but potentially serious brain side effect of nasal decongestants purchased from the High Street is currently being evaluated by experts.
Pseudoephedrine-containing ones are being examined because they might cause blood vessels supplying the brain to constrict or spasm, lowering blood flow.
The danger is that it might trigger seizures or even a stroke.
Drug regulators emphasize that there is very little chance that this will occur.
On patient information leaflets that are included with the medications, in addition to more widespread side effects like headache and dizziness, products already include warnings about the rare risks.
Anyone with questions about a medication should consult a doctor or pharmacist, according to experts. Every drug has the potential to cause side effects.
Pseudoephedrine is used to treat nasal congestion. It is available as sprays, liquids, and tablets and is occasionally combined with other medications for allergies, colds, and coughs.
Through its effect on blood vessels in the nose, the medication can aid in clearing blocked airways and reducing stuffiness.
Following a warning from French regulators about some recent, rare cases, the European Medicines Agency (EMA), which is also conducting a review, launched a nationwide review of pseudoephedrine in the UK.
The conditions posterior reversible encephalopathy syndrome (PRES) and reversible cerebral vasoconstriction syndrome (RCVS) will be examined by specialists.
Sudden, severe thunderclap headaches that last for days or weeks can be brought on by RCVS.
Confusion, headaches, seizures, and blurred vision are all potential PRES symptoms.
The UK's Yellow Card program, which is overseen by the Medicines and Healthcare Products Regulatory Agency (MHRA), allows people to report any alleged drug side effects.
The MHRA claims that it has only recently received a very small number of reports along these lines, including one PRES case in which the patient made a full recovery and one RCVS case in which the outcome was reported as unknown.
A spokesperson stated: "The safety of the public is our top priority, and we closely monitor the safety of all medicines to ensure that the benefits outweigh any risks.
"We are examining the evidence that is available. When necessary, we will offer any additional advice. .
"If you have any questions about your medication, please consult a medical expert. ".
The Royal Pharmaceutical Society's president, Prof. Claire Anderson, stated: "When new risks are discovered, patients may experience anxiety. It's right that they are being looked into by the proper authorities, and we're waiting to see how the EMA and MHRA reviews turn out.