Regulators are investigating the actions of Boots after the death of a great-grandfather who was sent duplicate packs of prescription drugs.
Richard "Tony" Lee took a week of extra pills dispensed in error shortly before his death in December 2016.
The General Pharmaceutical Council, which regulates pharmacies, said it was investigating his daughter's complaint.
Boots said "extremely rare circumstances" led to the error and has now apologised to the family.
However, the family is still waiting for a written apology from the company.
Mr Lee, 84, from Doncaster, was supposed to be sent four 108-pill, weekly packs each month to control heart, kidney and anxiety conditions.
But after his death, his daughter Gail Pickles found double the expected number of packs, along with evidence he had taken the double dose.
After reporting the duplicated medication packs found among her late father's belongings, she said she was promised there would be an investigation and "Boots would be transparent".
But trying to follow this up in February 2017, she was told the Nottingham-based company "could not comment at this time". Mrs Pickles said the "tone of the letter was shocking".
She was determined to look more closely at what had happened, a decision backed up in January this year when she watched a BBC investigation into Boots, which covered three patients who died after pharmacy errors.
In June, at the inquest at Doncaster Coroner's Court, it was revealed that when responsibility for sending the prescription was moved to a new pharmacy, the original pharmacy did not get a message to cancel its order, resulting in two sets of the same prescription being sent to Mr Lee.
Assistant coroner Louise Slater said excess dosing would have increased the risk of a cardiac event and was "a contributory factor" in his death.
Mrs Slater told the inquest shortfalls in communication had "resulted in a lack of clarity as to which pharmacy would assume responsibility".
"I'm satisfied that policies and procedures were in place at the pharmacies," she said.
"However, they were not followed on this occasion due an individual failing rather than a systemic failure."
Immediately after the inquest, Mrs Pickles said she was "appalled" by Boots' actions, particularly the revelation an internal investigation report into the error stated Mr Lee did not need medical attention and the incident did not need to be escalated.
Mrs Pickles from Penistone, South Yorkshire, also believes the Boots pharmacies had repeatedly failed to meet Boots' own rules - known as Standard Operating Procedures - for dispensing prescriptions.
After examining a total of 15 Medisure packs, including the duplicate ones, she said: "There's a space [on the pack] to write the 'week commencing...', had that been filled in alarm bells might have rung for Dad.
"They hadn't labelled properly, for example saying 'take so many times a day' but not how many tablets.
"Had they phoned as they're supposed to, to say a delivery's coming, [it may have brought the error to light but] they weren't doing that."
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Mrs Pickles said: "I don't think that any pharmacist goes to work thinking that they're going to cut corners and harm patients. They're going to work to do their job properly.
"Don't they have time to do it? Don't they have staff to do it?"
Dr Anthony Cox, from University of Birmingham Pharmacy School, who examined the packs for BBC Inside Out, said: "Pharmacists are horrified when they've made an error.
"If you are getting errors and violations occurring then you do need to look at your standard operating procedures, you do need to look at staffing, you do need to look at the workload pressures on people to see whether they're contributing to that."
But apart from the critical double dose, Dr Cox could not find an error which on its own could have caused Mr Lee harm.
Boots said it had apologised for the tone of the letter but denied it was not transparent - saying it could not comment further while inquest proceedings were ongoing.
It has also repeatedly expressed its condolences over Mr Lee's death, accepts it should have provided a written apology sooner and has offered to meet Mrs Pickles.
The firm insisted "rare and exceptional" circumstances led to the original mistake and it had an extremely low error rate.
Boots told the BBC the report revealed at the inquest was the initial report from the store which was based on limited information and a full investigation was already under way.
It added procedures had been updated and pharmacists were asked for feedback about them in 2016 to identify whether any additional support was needed.
Boots accepted there were some failings complying with standard operating procedures in this case but said they were not symptomatic of a wider problem.
During the Inside Out investigation in January, Boots said it was confident it had both the resource to deliver patient care and the company had enough pharmacy staff.
It also said it had contacted Dr Cox to discuss patient safety further and drive best practice.
Boots added it fully complied with the coroner's investigation.
The General Pharmaceutical Council, which in January cleared Boots of risking patient safety, said it was investigating Mrs Pickles' concerns and it took all complaints raised extremely seriously - in particular those relating to a patient death.
Mrs Pickles said: "My family were very, very wrong to place our trust in that company.
"If I hadn't looked through dad's medication, if I had just bundled them up and taken them back to the pharmacy none of this would have come to light."
Mrs Pickles is considering a civil claim.
You can see this story in full on BBC Inside Out East Midlands at 19:30 GMT on Monday on BBC One, or via iPlayer for 30 days afterwards.