What puzzles the Australian death tourists is why more Americans, who are so close to the border, don't take advantage of the easy availability of Nembutal.
"In the U.S., at least the middle and upper classes who can afford to travel usually work out some sort of behind-the-scenes arrangement with their doctor," said Dr. Art Caplan, director of the center for bioethics at the University of Pennsylvania.
"I write a prescription and you are not to take more than 30 pills," he said. "Doctors warn people, but there is a lot of winking and nodding."
Americans also use "guns and bridges," according to Caplan, who said typically doctors -- even in cases of the terminally ill -- try to avoid suicides and counsel on pain control and emotional support.
"As a culture, we seem to think that the medical way is better," he said. "Our society strongly condemns assisted suicide."
But Barbara Coombs Lee, president of Compassion and Choices, a nonprofit organization that works to improve care and expand choice at the end of life, said "desperate and determined" people will travel the globe to find a "peaceful and humane way out."
"Many people fear getting stuck in a prolonged and degrading process," she told ABCNews.com, but crossing the border to buy drugs designed for pets may not be the safest alternative.
"Ask any woman," she said. "The back alley is a dangerous and degrading place to get medical care. It used to be the VW bus and Dr. Kevorkian. Now it's the vet pharmacy."
Meanwhile, Don and Iris Flounders say their decision was made of sound mind and not in haste.
"It has to be well controlled, or everyone is going in to the chemist [pharmacist]," he said. "It's for people who have seen their doctor and have been thoroughly examined and mentally tested and cleared."
For them, knowing they have the Nembutal -- just in case Flounders' pain is out of control and his family is suffering -- gives them peace of mind.
"You don't feel the need to use it nearly as much as the longing for it," he said. And bringing back a dose for his friend has also given him comfort.
"Anjy said before, she resented dying," Flounders said. "Now, she says, 'You have given me life.' She didn't have a choice before."
The expensive and arduous trip to Mexico was "worth it," he said. And Flounders has not changed his mind about taking his life when the time is right.
"We are quiet-living people and not excessively excitable," said Flounders. "I'm the one who got the death sentence, but it doesn't worry me. As far as I'm concerned, I've had a good life."
ABC News' Karin Halperin in Mexico and Cloe Shasha and Gerard Middleton in New York contributed to this report.