Detectives investigating how packs of Nurofen Plus were found to contain anti-psychotic and anti-epileptic drugs are quizzing a man in custody.
Scotland Yard said the suspect, 30, from Bromley in south London, was arrested on Friday on suspicion of contamination of goods. He is being held in a south London police station.
Reckitt Benckiser, the drug's manufacturer, recalled the product on August 26 and halted distribution after discovering five boxes contained other companies' medicines.
It estimated some 250,000 packets were still in customers' hands.
Four of the affected 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.
The Met's Specialist Crime Directorate (SCD), which tackles serious and organised crime, is leading the probe and has been examining the entire production line of the drug as part of the inquiry.