Tasmania's Labor Premier Lara Giddings and Greens leader Nick McKim have lost their bid to give terminally ill people the right to take their own lives.

Debate on the contentious private member's bill, co-sponsored by Ms Giddings and Mr McKim, lasted almost 10 hours over two parliamentary sessions before it was put to the vote.

The legislation would have allowed terminally ill Tasmanians to end their lives 10 days after making three requests to their doctor.

In the closing stages of the debate, Ms Giddings made a last-ditch attempt to save the bill, urging pro-euthanasia Liberal MP Adam Brooks to change his mind and support it.

"Take us to the committee stages, allow us to sit with you, vote for the principle at the second reading," she said.

"Help us and get it to the committee stages so we can go through each part of this legislation and work through the issues you've raised."

The final vote, cast about 9:00pm (AEDT), was 13 against and 11 in favour, with all Liberal MPs and three Labor members opposing it.

Backbencher Brenton Best was one of the Labor MPs who did not back the law.

He took a swipe at both the Premier and Mr McKim.

"In some ways I think that the whole notion of a conscience vote started off on a poor footing, and I'm not proud to say that the leader of my party was part of that as well," he said.

"A statement that was made by both co-sponsors of the bill that called upon all members of Parliament to put politics aside - I don't need somebody to tell me that, how to use my conscience."

Labor MP Graeme Sturges voted against a similar proposal in 2009, but told the house he had changed his mind.

Cabinet minister David O'Byrne became emotional when reading out a letter from a constituent urging him to back the law.

"Gailene was referred to a palliative care doctor, who when he asked, 'What do you want to happen?', Gailene replied without hesitation, 'I just want it to be over'," he said.

"As Gailene's condition inexorably deteriorated, the dosage of medication increased to the point where if Gailene knew her family or I were present, she was unable to acknowledge us - basically she was comatose."

Liberal MP Adam Brooks said he personally supported euthanasia, but could not vote for the bill because it was badly drafted.

"If we're going to legislate for assisted dying, we should make sure we get it right."

Just before the vote, the Premier urged Mr Brooks to change his mind.

"We can go through each part of this legislation and work through the issues you raise."

Premier 'disappointed' by the result

Ms Giddings said polling had shown that 80 per cent of Tasmanians from all political persuasions supported law reform around euthanasia.

She expressed her disappointment that the bill was voted down.

"It is my view, and the view of many experts, that the legislation defeated today could have operated safely and effectively in Tasmania," she said in a statement.

"The bill was professionally drafted by the Office of Parliamentary Counsel following extensive research of international models and broad consultation with the community, including the medical profession.

"It is disappointing that those who sought to argue against the bill on technical drafting grounds, including the Law Society, did not seek to understand the good reasons why certain terminology was used."

Euthanasia advocate Phillip Nitschke says the bill's failure will not stop people from taking their own lives.

"They've waited for a long time, they're elderly and they're effectively saying the political process is not doing what we want," he said.

"They're voting with their feet, they're making their own choices, more people are importing the drugs.

"The internet has changed the whole face of the issue now and people, when they get their drugs, really don't care whether the law's changed or not."

The Dying with Dignity group's Margaret Sing believes the bill's strong safeguards were overlooked.

She says the result is upsetting for people who feel they have no other option.

"Where their doctors, two doctors at least, have agreed after a thorough examination...there were so many safeguards in the bill and it isn't reasonable to assume this bill was going to allow open slather."

"It just wasn't," she said.

Diana Hutchison from the group Real Dignity, which opposes legalising assisted suicide, says the defeat of the bill is a good result.

"There are, in fact, no safeguards that can protect the most vulnerable in our community."