The most chilling detail in “Better Off Dead?” a new BBC documentary by the English actress and comedian Liz Carr, is an automated telephone message.

Carr, who has been disabled since childhood and uses a wheelchair, made the film as Parliament debates whether to make it legal for doctors to help end the lives of people who are terminally ill and who say they wish to die. Similar legislation is being pushed in Massachusetts, where a bill permitting doctors to prescribe suicide drugs to patients with less than six months to live is now before the Senate Ways and Means Committee.

The film lasts just 58 minutes but it powerfully refutes the claim that authorizing medical professionals to facilitate the deaths of people with a fatal illness or disability is either enlightened or safe.

To show what legalization would mean in practice, Carr traveled to Canada, where “medical assistance in dying,” or MAID, was legalized in 2016. Like many countries, Canada has 24/7 suicide hotlines to prevent troubled people from acting on their suicidal urges. But it also has a hotline to expedite deaths. Dial the number and you get this message:

“Thank you for calling. The Ontario Medical Assistance in Dying Care Coordination Service is designed to provide information about end of life options in Ontario and referrals for Medical Assistance in Dying. If you would like to speak with an adviser to access the service or get more information, please press 1.”

What was once a trope of dystopian science fiction — government bureaucracies making it easy for despairing people to cut their lives short — is now Canada’s reality. That should be of interest in Massachusetts and the United Kingdom, where advocates for physician-assisted suicide keep insisting it will be restricted to adults of sound mind whose conditions are fatal.

In her documentary, Carr interviews one of those advocates, Lord Charles Falconer, who dismisses concerns that the criteria for assisted dying will inevitably be expanded. “Once a legislature decides it’s going to be terminal illness only, it will stick at that,” he says firmly. “The line in the sand for me is terminal illness. It goes no further than that.”

But the thing about sand, as Carr observes tartly, is that it shifts.

It certainly shifted in Canada. When MAID took effect in 2016, its boundaries were clear: Only mentally competent adults dying of a terminal condition could be approved for euthanasia. That year, 1,018 Canadians made use of the law to end their lives.

Yet soon the law was expanded to include anyone with a “grievous and irremediable medical condition,” whether fatal or not. The number of MAID deaths skyrocketed. There were well over 13,000 in 2022. And under a new legal provision scheduled to take effect in 2027, Canadians suffering from mental illness, even if they’re physically healthy, will have the right to be “assisted” to their deaths. That is already happening in the Netherlands, one of the few other countries where physician-assisted suicide is lawful. The Telegraph reported last week on the soaring number of Dutch residents who have been euthanized because of a psychiatric disorder. Those choosing to have their lives ended in this way have been as young as 16.

Among the most eloquent opponents of assisted-suicide laws are disabled activists who fear that once people are allowed to seek a doctor’s help in ending their life, many will be coerced into doing so. That coercion might come from family members unwilling to bear the strain of long-term medical care when assisted suicide is a cheap alternative. It might come from insurance companies refusing to reimburse the cost of lifesaving treatments once legalization lets them offer aid in dying as a covered benefit instead. It may come from physicians, many of whom, recent research shows, underestimate the quality of life of individuals with significant disabilities.

Popular culture and the news media often portray assisted suicide as dignified, compassionate, even heroic. Carr was galvanized into action by the 2016 Hollywood romance “Me Before You,” in which an athlete paralyzed in an accident chooses to cut life short through assisted suicide and not burden the woman he loves. The film’s promotional hashtag was #LiveBoldly.

With assisted suicide increasingly glorified like this, it’s no wonder public opinion now supports it. In a new UMass Amherst/WCVB poll, 67 percent of respondents said they favor legalizing medical aid in dying for terminally ill patients. At a State House rally last week, lawmakers expressed confidence that doctors in Massachusetts will soon be allowed to prescribe death. Anyone who imagines that would be a good thing ought to take an hour to watch “Better Off Dead?”