LONDON (Reuters) - British regulators issued a safety alert on Thursday after discovering packs of the over-the-counter painkiller Nurofen Plus containing the anti-psychotic drug Seroquel XL.

The Medicines and Healthcare products Regulatory Agency (MHRA) issued the alert, potentially affecting thousands of packs, after receiving reports of rogue Seroquel XL tablets in three different batches of Nurofen Plus.

The two medicines look very different but the MHRA said it had been informed that two patients took Seroquel XL instead of Nurofen Plus as a result of the unexplained packaging mix-up.

Seroquel XL, made by AstraZeneca, is a prescription-only drug used to treat serious psychiatric disorders including schizophrenia and bipolar disease. Nurofen Plus, made by Reckitt Benckiser, is a popular painkiller containing codeine that needs no prescription.

Neal Patel of the Royal Pharmaceutical Society said a single dose of Seroquel XL was unlikely to cause major problems for a healthy adult.

"However, for patients taking other medicines or those with other medical conditions or if you suspect you may have accidentally taken Seroqel and feel unwell, seek advice from your pharmacist or doctor as soon as possible," he said.