The medicines regulator says some batches of Nurofen Plus contain blister packs of Seroquel XL, a prescription-only drug used to treat schizophrenia and bipolar disorder.
Nurofen’s manufacturer, Reckitt Benckiser, says there have been three cases identified so far, all in south London.
It says “serious investigations” are under way to establish how the mix-up occurred, but sabotage is considered unlikely.
Pharmacists have been advised to check before selling packs of Nurofen Plus, a painkiller that contains codeine.
Seroquel is manufactured by a different drug company, AstraZeneca PLC.