The programme, set in Liverpool, showed the terminally-ill Gladys Charlton begging members of her family to end her suffering after her GP had failed to describe sufficient morphine.
In last Tuesday's episode, her daughter, Elaine, and son-in-law, Mick, were shown smothering her with a pillow.
Channel 4 set up a helpline for worried viewers as doctors said the programme had undermined the confidence of families caring for terminally-ill relatives.
Dr Rob Barnett, who works in Liverpool, said: "I am quite disgusted by it. It casts a slur on Liverpool doctors. I cannot imagine any self-respecting GP treating a family in the way this family has been treated."
Families of patients with cancer being treated in hospitals in the city had been upset by the programme because it was not true to life, Dr Barnett said. He had been angered by a scene in which a GP was shown giving Gladys an injection of morphine which had worn off, leading Mick to buy heroin from street drug dealers which he then injected into Gladys himself.
"I cannot believe that the situation as portrayed is a credible one in this country," he said.
Dr Bill O'Neill, of the British Medical Association, said the programme had shown the GP telling the family that Gladys had been given as much morphine as allowed.
"That is not true. There is no limit. The amount of morphine needed is the amount necessary to control pain," he said.
Although in the past doctors had feared that giving large doses would lead to addiction, there was now a recognition that the risks of addiction were lower than had been thought and in terminal cases it did not matter anyway.
"We have better drugs now and they are given in more appropriate doses," Dr O'Neill said.
He said the programme also showed the family denied support during an emotionally stressful time and given no opportunity to discuss their anxieties.
In fact, a nationwide network of nurses trained in the terminal care of cancer patients was now available.
Professor Gordon McVie, director general of the Cancer Research Campaign, said the idea that families could be forced to obtain drugs on the street and perform euthanasia on a relative was "not only unreal, but irresponsible".
The storyline was gloomy and negative and would frighten those living with cancer or treating relatives with the disease, he said.
Professor Flora Finlay, of the Marie Curie cancer charity, said: "For a TV programme to give the message that there is no pallative care available for cancer patients is totally misleading" she said.
Brookside executive producer Phil Redmond said: "One of the biggest problems facing our society is the care of the elderly.
"It seems an important and legitimate concern for a drama to plot the mental, medical and intellectual issues concerned with death."
A spokeswoman for Channel 4 said 350 people had called the help-line in the three hours after the show was broadcast.
Half had complained, but half said it showed what had happened to their relatives.Reuse content