The Medicines and Healthcare products Regulatory Agency (MHRA) has issued the drug alert following reports that batches of Nurofen Plus contain individual blister packs of another drug, Seroquel XL 50mg.
The government agency says that three defective packs have been found in pharmacies across the UK, while thousands more could potentially be affected.
The affected packs of Nurofen Plus, which all contain 32 tablets, carry the following batch number, expiry date and product licence number:
13JJ 03/2014 00327 / 0082
57JJ 05/2014 00063 / 0376
49JJ 05/2014 00063 / 0376
Each batch contains between 4,000 and around 7,500 packs, amounting to around half a million tablets. However, not all packs are affected.
The large capsule-shaped Seroquel XL 50 mg tablets can be identified by their gold and black packaging, while the smaller Nurofen Plus tablets can be identified by their silver and black packaging.
Ian Holloway from the MHRA's Defective Medicines Report Centre (DMRC) said: "People should check to see if they have any affected packets of Nurofen Plus. If you do, return them to the pharmacy where you bought them from. "
"You can also report this to the MHRA's DMRC on 020 3080 6574.
"If you have taken a tablet and you have any questions, speak to your GP."
In a statement, Reckitt Benckiser, manufacturer of Nurofen Plus, said the three cases identified so far have all been confined to south London.
It said "serious investigations" are under way to establish how the mix-up occurred, especially as 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," the statement said.
"We are taking this matter very seriously and are working closely with the regulatory authority, the MHRA, and pharmacies.
"The MHRA are investigating the issue, and have considerable law enforcement powers.
"Nurofen Plus is a pharmacy-only medicine which means it is behind the pharmacy counter.
"It is not available for self-selection from the shop floor - and therefore pharmacists are able to check packs and greatly reduce the likelihood of affected packs being sold."
Dr Aomesh Bhatt, medical director for Nurofen Plus, said: "We are in the process of working to ensure the Nurofen Plus packs are double-checked by pharmacy staff before they are handed to customers.
"We encourage consumers of Nurofen Plus to be vigilant and, while it is very unlikely, should they find they have a suspect pack or if they have any other concerns, we advise them to speak to the pharmacist where they purchased the product."
Seroquel XL is a prescription-only anti-psychotic drug used to treat several disorders including schizophrenia, mania and bipolar depression.
A spokeswoman for AstraZeneca said people who have mistakenly taken the drug should contact their GP and take the medicine back to the pharmacy where they purchased it.
Side effects of the drug include dizziness, headache and sleepiness.