The BBC, wrote in a section titled "Anti-Euthanasia Arguments" in its "Ethics Guides" page of (accessed July 26, 2018):

“Many people worry that if voluntary euthanasia were to become legal, it would not be long before involuntary euthanasia would start to happen…

This is called the slippery slope argument. In general form it says that if we allow something relatively harmless today, we may start a trend that results in something currently unthinkable becoming accepted.

Those who oppose this argument say that properly drafted legislation can draw a firm barrier across the slippery slope.”

July 26, 2018