The hunt is continuing for a potential saboteur after packs of Nurofen Plus were found to mistakenly contain anti-psychotic and anti-epileptic drugs.
Reckitt Benckiser, the drug's manufacturer, recalled the product on Friday night and halted distribution after discovering five boxes contained other companies' medicines.
It has estimated some 250,000 packets are still in customers' hands.
Scotland Yard launched a probe after four boxes were found to contain the anti-psychotic drug Seroquel XL 50mg and one packet contained Pfizer's anti-epileptic medication, Neurontin, in 100mg capsules.
Its Specialist Crime Directorate, which tackles serious and organised crime, is now investigating.
Examinations will cover the entire production chain in a bid to identify how some packs of Nurofen Plus came to contain strips of the potentially harmful drugs after Thursday's safety alert.
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."
Ian Holloway, of the Medicines and Healthcare products Regulatory Agency's Defective Medicines Report Centre, said: "People should check to see if they have any packets of Nurofen Plus. If you do, return them to your nearest pharmacy."
Seroquel XL is a prescription-only anti-psychotic drug used to treat several disorders including schizophrenia, mania and bipolar depression. People who accidentally take the drug may experience sleepiness and are advised not to drive or operate any tools or machinery.