The mother had taken two slices from the pack of Roberts wholemeal bread – which she had bought from a local Tesco store – when she made the shocking discovery.
Even more shocking came the revelation that the dirty needle had been pushed through the loaf's plastic wrapper by a grandad.
A court heard that David Rodgers, 61, had been desperate to dispose of the needle so his wife wouldn't know he was back on the drug.
He had already used it for a heroin 'fix' before he hid it in the bread in Tesco Extra, Pendlebury, Lancashire, on December 1.
The 'alarmed and distressed' mum handed the needle in to police, who used DNA tracing to track down Rodgers. He pleaded guilty to contaminating the loaf with intent to cause public alarm at Manchester magistrates' court.
The case was adjourned for pre-sentence reports.
Rodgers – of Cumbrae Gardens, Salford – was warned he could face jail. Dominic Geelan, prosecuting, said Rodgers had kept the needle in his pocket for two days after injecting himself with heroin. There was no suggestion Rodgers had any sort of grudge against Tesco.
Rodgers had told police: "It's hard to believe what I did, being a father and grandad."
Mr Geelan added: "The only reason he could think of was that he didn't want his wife to know he was using heroin again."
A Tesco spokesman said: "We have been working closely with the police throughout their investigation and on behalf of our customers are pleased that this has been resolved."