Sheila Nicholson, 64, from Beckett Avenue, Gainsborough, died at her home on 3rd January.
But the hearing at Lincoln Cathedral Centre revealed the extra medication didn’t attribute towards her death.
The post-mortem showed 44 milligrams of morphine was injected into her body through a syringe driver after a ‘calculation error’ by an out-of-hours Lincolnshire GP.
Newly qualified Dr Chhaya admitted fault and has been disciplined. He received a written warning, which will stay on file for 12 months, and will have further training.
A nurse, who was aware of the mistake, also received a written warning, which will remain on her file for a year.
Coroner for West Lincolnshire Stuart Fisher said it was not being treated as a criminal matter.
The family, who were present at the inquest, were visibly upset as the coroner began to summarise.
“The post-mortem showed death was attributed to cancer not the prescription or administration error,” said Mr Fisher.
“Your mother was a relatively young woman by today’s standards. It is tragic she should lose her life in such a way.”
General Manager for Lincolnshire Community Health Services Catherine Wylie confirmed a full investigation was conducted following the incident.
“We offer our sincere condolences to the family of Ms Nicholson who died in January,” she said.
“A full investigation was conducted at the time and appropriate action has been taken with staff into their performance and capability. As an organisation we are committed to learning from any incident and reviewing our procedures to avoid any errors.”
Ms Wylie added the service has guidelines in place that cover all aspects of prescribing and dispensing of medicines.
“In addition, we provide further guidance through our controlled drugs policy on the prescribing, dispensing and administration of controlled drugs,” she said.
Mr Fisher returned a verdict of death by natural causes.