Washington will become only the second US state to allow assisted suicide on Thursday as its new "death with dignity" law takes effect.
Under a ballot measure passed during November elections, physicians in the northwest state will be allowed to write prescriptions for lethal doses of drugs for terminally ill patients who have less than six months to live.
Only one other US state -- Oregon -- has similar legislation, although a court in Montana recently ruled that terminally ill patients had the right to seek physician-assisted suicide.
Supporters of the new Washington legislation, made possible by a 2006 US Supreme Court ruling, say the law allows for "aided dying" rather than assisted suicide or euthanasia.
"Aided dying is neither euthanasia nor suicide," said Terry Barnett, president of the Washington branch of Compassion & Choices advocacy group.
"It's not euthanasia because euthanasia implies action by a physician to end a patient's life. It's not suicide because people who choose aid in dying are not choosing to end their lives.
"They don't want to die -- they're choosing to end suffering that cannot be relieved and suffering that they are experiencing that is worse than death."
Supporters of the law say a string of stringent checks and balances have been put in place to prevent the system being abused, rejecting critics who argue the law will make it easy for sufferers to choose to die.
Under the legislation any patient seeking a prescription for lethal drugs must be 18 years old and a permanent resident of the state.
To obtain a prescription a patient would need to make two oral requests followed by a written request witnessed by two people who are not related or connected to the individual. Two doctors must also affirm that the patient has six months or fewer to live.
Barnett said Washington physicians who object to the legislation have the right to decline a request for a lethal prescription.
"Anyone who has an objection of conscience may choose not to participate," he said. Physicians were also not allowed to administer the fatal drugs.
"Under our law a person has to self-administer medication," Barnett said. "All a physician can do is write a prescription as he or she would for any kind of medication, except that there are numerous safeguards that are required for aided dying medication that are not required for other medications."
Barnett said the experience of neighboring Oregon indicated Washington was unlikely to witness a surge in terminally ill people seeking to take advantage of the new law.
"The experience in Oregon has been that when someone gets a terminal diagnosis and told that they have six months to live, they don't rush right out and get a prescription," he said.
"People wait until their situation has got very difficult, then they get a prescription, and they tend to hold on to it. Actually having the prescription enables people to keep going and not end their lives.
"Around 30 percent of people who get prescriptions don't take them and end up dying of their underlying illness.
"We know from these people that what was important was having the knowledge that they had the option if things became too difficult. Having that knowledge allowed them to keep going."