An expert witness has told the High Court legalisation of assisted suicide would not introduce the practice, but instead would regulate and control something already in every jurisdiction.

Professor Margaret Battin was giving evidence by video link from the University of Utah in the case of Marie Fleming, the 58-year-old Wicklow woman who is challenging the law on assisted suicide.

Professor Battin, who is an expert in medical ethics, told the court practice of assisted death had always existed prior to legalisation.

She said there was no evidence from studies in the US and the Netherlands that legalised assisted suicide led to abuse of vulnerable people such as the elderly, poor or disabled.

She also said the various descriptions of palliative sedation obscure the reality of what actually happened.

Professor Battin said in many jurisdictions in order to deal with intractable pain, patients were sedated to a lowered or unconsious state and kept that way.

They cannot drink or eat and often they were not hydrated or fed and died of dehydration or starvation.

However in cross examination she agreed she did not know what the practice was in Ireland but this was the case in most developed health care systems.

When told by Counsel for the State Michael Cush that this was not the practice in Ireland, Professior Battin asked: "Are you certain?" He replied: "I'm certain that those are my instructions."

She said she could not see how a physician who introduces sedation without hydration and says it is merely intended to relieve suffering without also recognising death will be the result of the procedure.

She said palliative care and assisted suicide should both be options for the end of life and should not be in a "turf war".

She said adequate safeguards are in place in the state of Oregon and in the Netherlands for end of life deicisions.

Only 0.2% of people who die in Oregon die in this way and only 3% of people in the Netherlands, she said.

It still remained a psychologically important option.

She agreed that other opinions had been written that the Oregon Death With Dignity Act may not adequately protect patients with mental illness or depression which in some cases may be missed or overlooked.

However she said she noted the word "may".

She said she had no data on the practice of illegal assisted suicide in Ireland but suggested it was no different to any other jurisdictions.

The reason the question was so important was the dramatically increasing liklihood of facing circumstances like this as modern medicine was now able to keep people alive and diseases had long and predictable courses.

She said Marie Fleming's case was far more extreme but this was part of what it is to die in the developed world.

Earlier, the court was told the Constitution does not confer on citizens a right to commit suicide.

The submission was made by lawyers for the State.

Senior Counsel for the State Michael Cush said it was a radical proposition to ask the court to recognise a right which is not expressed in the Constitution.

The Constitution conferrerd no right on anyone to commit suicide and the policy of the law remained firmly adverse to suicide, he said.

He said if there was a right to commit suicide it would be logically impossible to have a law against aiding and abetting suicide.

"Society disapproves of suicide and all its terrible consequences for family, community, economically and socially," he said.

While suicide had been decriminalised it did not mean there was a right to commit suicide, he added.

And while Marie Fleming was not looking for a general right to suicide, her submission that it should be available to a certain class of people was even more radical and was not for the court to decide, he said.

"Instinctively the court should feel an enormous reluctance to trespass on the making of those decisions."

Mr Cush said previous cases on the right to refuse medical care and to die a natural death were very different to what the court was being asked to do in this case.

Marie Fleming was diagnosed in 1986 with MS and is in the final stages of the condition.

She is past the point where she could take her own life unaided but wants to establish the right to end her life with assistance.

She is challenging a section of the Criminal Law Suicide Act which makes it an offence to help someone take their own life. She claims the section of the act is contrary to the provisions of the Constitution and incompatible with the European Convention on Human Rights.

She has asked the court for a declaration that the section of the act is invalid. Failing that she wants the DPP to outline the criteria used in deciding on a prosecution in cases of assisted suicide.