A disabled man took the wrong medication because it had been incorrectly labelled at a Brent pharmacy.
Andrew Coleman, 40, was forced to seek medical advice from a doctor after taking antibiotics, which he says should have been for his partner.
It later emerged incorrect labels had been put on three prescriptions, including a box of temazepam, at the Asda pharmacy in Forty Lane, Wembley.
Mr Coleman, of Chalkhill Road, Wembley, is furious with the super-market chain and has made an official complaint to the Royal Pharmaceutical Society of Great Britain, the watchdog which investigates pharmacies.
He said: "Because my partner and I are disabled my friend put the prescriptions in and picked them up as usual. It wasn't until later that I realised all the medications had my name on when some should have been for my partner.
"By that point I had already taken antibiotics that were not supposed to be for me. The following day my friend went back to the pharmacy and explained what had happened. The pharmacist looked at the receipts and the wrong labels and agreed this was a very serious offence."
Asda sent Mr Coleman a letter of apology and £150 in vouchers to spend at the store. However, when he returned to spend the vouchers he was accused of shoplifting.
He said: "I was so embarrassed. Not only did they get my prescriptions wrong but then I was accused of shoplifting. Nobody would ever want to be branded a common thief."
And Mr Coleman says this is not the first time Asda has mixed up the labels. He said: "It is outrageous. This is the second time this has happened. Because of the laid-back attitude of the store I could be taking the wrong medication, which is extremely dangerous.
"I want to make sure people are aware that this kind of practice is taking place at Asda and I would be happy to attend a hearing and give evidence on the matter. I hope it is investigated."
An Asda spokeswoman said: "Asda apologises to Mr Coleman for any inconvenience caused to him or his wife. Mr Coleman received £150 compensation, which he accepted at the time and the matter was settled.
"Mr Coleman has suggested this isn't the first time this has happened to him. However, there is no previous record of this happening."