Sonia Still, 27, consulted her GP after noticing she had been handed a drug containing morphine for her eight-month-old son Harry.
She says her doctor confirmed he had prescribed Orciprenaline Syrup - which is suitable for infants - and not Oramorph Oral Solution, which contains morphine.
Three years ago the same Lloyds pharmacy in Magpie Crescent, Stevenage, Hertfordshire, was also accused of giving a mother the wrong prescription for her youngster.
Mrs Still was about to give her baby his medicine when she read the accompanying leaflet and "alarm bells" began to sound.
She said: "How the pharmacy gave this to me is beyond me.
"They must have realised it was for a child because I had ticked the box for the person being under 16 and I didn't have to pay.
"I was a young mother there with two young children so it should have been obvious to someone."
Lloydspharmacy held an investigation after the incident three years ago and an inquiry was also carried out by the Royal Pharmaceutical Society of Great Britain.
The society concluded the pharmacy was well-run and the incident was a one-off.
However, Lyn Boreham, whose son Daniel was wrongly given a morphine-based drug in the incident three years ago, is calling for action.
She said: "Something has got to be done.
"In 2000, I was told it was an isolated incident and procedures were in place to ensure that it didn't happen again. But it has happened again."
'Sincere apologies'
Lloydspharmacy said it would be contacting Mrs Still to discuss the incident and an investigation would take place.
Deputy superintendent pharmacist for Lloydspharmacy Nick Mortimer said: "Lloydspharmacy take incidents concerning incorrect dispensing of a prescription extremely seriously.
"We are committed to providing a high standard of health care services to patients.
"We would like to offer our sincere apologies to the family involved."