Under the law as it stands, some people die in pain without the dignity they had hoped for. So began a leader in The Herald last year about the campaign by Independent MSP Margo MacDonald to legalise physician-assisted suicide for the terminally ill. It briefly summarised both sides of the argument but the perceptive reader could easily tell that the author was quietly cheering on Margo. I (yes, I wrote it) made the logical point that the law as it stands discriminates unfairly between those capable of taking their own lives (as suicide is no longer illegal) and those so incapacitated as to require assistance from another person, who, in theory at least, could then be prosecuted for culpable homicide in Scotland (or abetting suicide in England and Wales). Paradoxically, removing this threat could prolong life because patients would no longer feel obliged to take action while still capable of committing suicide unaided.

This week Lord Falconer attempted to relax the law on assisted dying through an amendment to the Coroners Bill. Though only applicable south of the Border, success would have altered the climate for Margo MacDonald's End of Life Choices (Scotland) Bill as it goes to committee at Holyrood later this year. In a free vote in the Lords, Falconer failed by 191 votes to 141. Disappointed? No, I was hugely relieved.

Why have I changed my mind? The leader writer in me would carefully marshal the intellectual arguments against reform and harvest some key quotes from medical and religious leaders but that would be fundamentally dishonest because this change is primarily heart-felt.

It may seem banal to start with Tigger but you often hear people saying of a loved one that they "wouldn't let a pet suffer like that". When our 20-year-old ginger moggy developed a large tumour last year and had difficulty walking, we eventually decided to "have him put to sleep". Our gentle vet did the deed but it was our decision, our responsibility. I held and stroked him and felt him flinch as the fatal dose took hold before he relaxed in death. A year later, every time I see the final photo of him sunning himself on the lawn half an hour beforehand, that moment haunts me. The idea of taking such responsibility for the death of a relative now seems inconceivable.

Now, like Margo MacDonald, my father has been diagnosed with Parkinson's disease. He suddenly seems terribly vulnerable. There is much less of him to hug. He hates not being able to mow the grass any more or drive. "I feel so useless," he said the other day. It's as if we have silently changed places. It hasn't come to this yet but I can easily imagine him summoning the family and telling us brightly that he has "had enough" in the completely mistaken belief that he has become a bothersome burden. He is clever enough not to put it in those terms and could easily persuade two doctors that he has the "capacity to make a declaration", as the Falconer amendment requires. How could they or we distinguish between his stated wishes and his true motives?

At the end of life, people change and settle for less. As a senior civil servant, my late father-in-law would have stated categorically that he would rather be dead than live with double incontinence. Yet eventually he did just that, after several strokes and, well cared for in a local home, he continued to relish chocolate, Burns songs, the chatter of his grandchildren and his view of the Lomond hills for several years. Now those years seem precious. Yet in a care regime struggling to cope with rising numbers of frail elderly people, it is easy to imagine that the much- heralded "safeguards" against the abuse of assisted dying could become steadily degraded.

Lord Falconer said those taking relatives to the Dignitas clinic in Zurich faced a "legal no-man's land" and yet he seeks a clarity that simply doesn't exist. As the mother of a medical student, I'm starting to understand that medical diagnosis is not an exact science and even terms such as "terminally ill" are hard to define. Post-mortems often reveal misdiagnosis of terminal illnesses. How often are people told they have months to live, then survive for years? And how many terminally-ill people say they would be better off dead, not because their suffering is intolerable but because they are (quite justifiably) depressed?

My own Damascene conversion on this issue happened when watching an intelligent and sensitive televised debate between Margo MacDonald and Mario Conti. Both put their cases persuasively but the Archbishop won my support with an argument that owed more to Hippocrates than the Bible. Allowing physician-assisted dying would turn on its head a noble profession committed to sustaining life and relieving suffering. Wouldn't it have a subtle and deleterious effect on the doctor-patient relationship?

Then there is what has been described as "the darker side of some family dynamics". In a culture rife with elder abuse, it is unlikely that these decisions would always be made from altruistic motives. How many vulnerable elderly people would be talked into believing that they would be better off dead by relatives gasping for freedom or an early inheritance? Patients may then voice these sentiments to doctors as if they were their own. Relaxing the law effectively would give these relatives a "get out of jail free card".

That's why the possibility of prosecution must be retained, even though, as a Court of Appeal ruling earlier this year made clear, most families resorting to the Dignitas option have nothing to fear. They are unlikely to be prosecuted and, if they were, juries would be unlikely to convict them. Ultimately, public safety trumps the issue of choice.

Sometimes there is an argument for a legally grey area, as when doctors hasten the departure of clearly miserable patients obviously close to death, under the guise of relieving symptoms, something known as "the double effect". Even that must be the exception rather than the rule.

Interestingly, neither the BMA nor charities representing disabled and terminally-ill people have campaigned for a change in the law. In 2004, a group of these patients formed Not Dead Yet UK. A moving speech from their convener, Lady Jane Campbell, who has spinal muscular atrophy, in the Lords on Tuesday ensured the defeat of Lord Falconer's amendment. Despite "ticking all the boxes" for an assisted suicide, she told peers: "The last thing I want is for you to give up on me especially when I need you most."

So, Dad, if you read this, please know that I don't want to help you to die. I want to help you to live.