No-one has been prosecuted for helping them - despite the possibility of up to 14 years in jail - but doubt has lingered because some relatives were questioned by police.
Thursday's ruling does not give Ms Purdy's husband Omar Puente, immunity from prosecution.
But it means the DPP must now make clear the factors that would be taken into account when deciding whether to take action against relatives in such cases.
She told BBC Radio 4's Today programme there must be a debate to ensure the law distinguishes between malicious and compassionate acts, ensuring suitable safeguards are in place.
She said: "We can't allow de facto changes in the law. It's got to be the result of proper open discussion.
"The DPP has got the possibility of making sure that financial gain is definitely not something which should be allowed for assistance."
But she added: "I don't think there's going to be a rush to get 'Auntie May' to the knacker's yard because they want to inherit her house," she said.
Ms Purdy said the ruling would not mean people were compelled to take their own lives but they would have choice, pointing to evidence from countries where doctors can prescribe fatal doses of medication to terminally ill patients.
In the US state of Oregon, she said, of 97 people prescribed the dose, 50 died natural deaths.
However, Baroness Finlay, who chairs the All Party Parliamentary Group on Dying Well, said there had been cases in Oregon of cancer patients being offered funding for suicide but not for treatment.
"I would hate anyone to think that suicide is the answer to a problem," she said, adding that patients should be able to make informed choices about care rather than feeling "they would be better off dead".
However, she said clarifying the law would be helpful to patients and families alike.
Lesley Close, who helped her brother John, 54, from Milton Keynes, agreed.
After helping him die in Switzerland, she wrote to police asking whether they would prosecute her because she wanted to apply for jobs.
"The answer was no, it wouldn't be in the public interest to prosecute," she said.
"But I still didn't know what it meant, what I'd done that could be counted as a crime. It's been such a mess," she added.
Liberal Democrat peer Lord Goodhart said the law should go further so that people did not have to go "far away from their home and families" to take their own lives.
"It is going to be important to extend the law, not simply to make it a defence that you can go to Switzerland because that's where it's already legal."
However, Dr Andrew Ferguson, from the group Care Not Killing, said "a vocal minority" should not hold too much sway in changing a law "in a way that would affect profoundly the lives of far greater numbers of vulnerable people".
Debbie Purdy is concerned her husband could be prosecuted
David Morris, of Independent Living Alternatives, also has concerns.
"If you look at places where assisted suicide has been legalised, you will find that the culture and value of the lives of disabled people is lessened," he said.
Phyllis Bowman, executive officer of Right To Life, added: "We are extremely concerned about the manner in which this [ruling] will leave the vulnerable - that is the disabled, the sick, and the aged."
Meanwhile, MS Society chief executive Simon Gillespie said: "There is far more to living with MS - even in its more severe forms - than planning how to die."
Ms Purdy was diagnosed with primary progressive multiple sclerosis in March 1995. She can no longer walk and is gradually losing strength in her upper body.
She had previously lost challenges in the High Court and Court of Appeal. The Lords ruling was her last chance of success in the UK legal system.
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