Users of the pain reliever Nurofen Plus have been warned to check packets carefully after reports that they may contain anti-psychotic drugs.

Individual blister packs of Seroquel XL, used to treat schizophrenia, mania and bipolar depression, are thought to have been mistakenly inserted into Nurofen packs at a wholesaler. Thousands of packs could potentially be affected. They have been found in pharmacies across the UK, prompting the Medicines and Healthcare products Regulatory Agency (MHRA) to urge vigilance.

People who accidentally take Seroquel may become drowsy and are advised not to drive or operate machinery until they know how the tablets have affected them. The MHRA is advising anyone who thinks they may have taken one to contact their doctor.

The affected packs of Nurofen Plus all contain 32 tablets and are from batch numbers 13JJ, 57JJ and 49JJ. Each batch contains between 4,000 and 7,500 packs. Not all packs are affected.

The large capsules of Seroquel XL 50mg tablets have gold and black packaging, while Nurofen Plus tablets are smaller and have silver and black packaging.

Reckitt Benckiser, manufacturer of Nurofen Plus, said three cases had been identified so far, all in south London. "Serious investigations" were under way to establish how the mix-up occurred, it said. Seroquel XL is manufactured by another drug firm, AstraZeneca.

"After careful review of the manufacturing system, manufacturing errors by the makers of Nurofen Plus or Seroquel XL are not thought to be part of the cause at this stage," a statement said.