Darshan Arora, 66, was rushed to Fiona Stanley Hospital at 10am on Sunday.
The grandfather, who is already battling stage five kidney failure, was coughing and struggling to breathe.
He waited in emergency for four hours before he was seen and diagnosed with pneumonia.
His family said at 6am on Monday nurses mistakenly gave him 30 milligrams of blood pressure medication, six times his usual dose.
Family friend Philip Couper said staff told them they were “understaffed”.
“There were a lot of things going on and the hospital was experiencing a number of issues,” Couper said.
“But by that stage the damage was already done.”
Western Australian Health Minister Amber-Jade Sanderson said the staff involved haven’t been stood down but she is seeking answers.
“I’m not going to throw anyone under the bus,” Sanderson said.
“There’s an investigation that is due diligence for the family and the staff.”
Relatives said despite his illness Arora had been getting by without dialysis, but because of the suspected bungle his kidneys had been further damaged.
They said he will require being hooked up to a machine on a regular basis and his quality of life will be significantly reduced.
Fiona Stanley Hospital acknowledged a “difficult time for the family”.
“We will continue to directly engage with them on the patient’s care,” a South Metropolitan Health Service spokesperson said.
Janet Reah from the Australian Nursing Federation said nurses were “stretched so thin”.
“These incidents will be happening over and over again due to the staff shortages,” Reah said.
The suspected overdose came right before the inquest into Aishwarya Aswath’s death.
The seven-year-old died from sepsis while waiting for two hours at Perth Children’s Hospital.
“We need to fix our hospitals we need to fix the problems,” Couper said.
“You don’t expect to go into a hospital and let alone get worse.”