The makers of Nurofen Plus have recalled all remaining stock, saying sabotage is suspected in some packets.
Earlier, the Medicines and Healthcare products Regulatory Agency (MHRA) issued an alert after three packs were found to contain an antipsychotic drug.
Reckitt Benckiser (UK) Ltd said it had heard reports of the wrong drug in two more batches of Nurofen Plus.
A spokesman said the firm was working with police on a formal investigation to find those responsible.
Consumers are advised to return any packs of Nurofen Plus to a pharmacy.
Four packs were found to contain Seroquel XL 50mg. The packs were bought in Victoria, Bromley and Beckenham in south London.
Another tampered pack, containing a prescription medicine for epilepsy, was uncovered in Northern Ireland.
Most people who take one Seroquel will just experience sleepiness, but those on sedatives could feel stronger effects.
Thousands of 32-tablet packs are in the affected batches - numbered 13JJ, 57JJ and 49JJ.
Packs from the three batches - which contain between 4,000 and 7,500 packs each - have been distributed across the UK. But it is known that not all the packs are affected.
Reckitt Benckiser (UK) Ltd said one of the two additional batches affected contained Seroquel XL 50mg, and one contained the Pfizer product Neurontin 100mg capsules, which is an epilepsy drug.
Dr Aomesh Bhatt, medical director for Nurofen Plus, said: "We are taking this matter extremely seriously and have decided to recall all packs of Nurofen Plus as the most prudent course of action in the current circumstances.
"We're asking consumers to return any packs of Nurofen Plus to a pharmacy. No other Nurofen products are affected or being recalled."
A Reckitt Benckiser (UK) Ltd said: "Sabotage is suspected and we are working with the police on a formal investigation to find the person or persons responsible.
"Distribution of Nurofen Plus has been halted at this time."
Seroquel XL is a prescription-only anti-psychotic drug used to treat several disorders including schizophrenia, mania and bipolar depression.
Nurofen Plus is for pain relief and contains codeine and ibuprofen.
The Seroquel tablets are larger and have gold and black packaging, compared with the Nurofen Plus tablets which are smaller and have silver and black packaging.
Nurofen Plus is stocked behind the counter in chemists, so people cannot simply pick it up. Pharmacists have been asked to check the stock on their shelves.
Earlier, Ian Holloway, from the MHRA's defective medicines report centre (DMRC), said people who had affected packs of Nurofen Plus could also report it to the MHRA's DMRC on 020 3080 6574.
He added: "If you have taken a tablet and you have any questions, speak to your GP."
Two people are believed to have taken Seroquel by mistake, but are not thought to have experienced any ill-effects.
Professor David Nutt, head of the department of neuropsychopharmacology and molecular imaging at Imperial College, said: "The effect of taking Seroquel entirely depends on the dose.
"The only likely impact of a single ingestion would be sedation, but in people taking antihistamines and other sedatives the added effects could be quite extreme."
Neal Patel, from the Royal Pharmaceutical Society, said: "For a healthy adult a single dose of Seroquel is unlikely to cause major problem, however, for patients taking other medicines or those with other medical conditions or if you suspect you may have accidentally taken Seroquel and feel unwell, seek advice from your pharmacist or doctor as soon as possible."