"It has inflammatory effects and helps breathing," he said. "It's an expectorant and brings the mucus up. Some people even say their lung function tests better, at least temporarily. It's hard to tease out what is psychological and what is long-term.
"I do think it should be used broadly and for more people, but the fact that it's moving in this direction is very encouraging," he said of the New Jersey law.
But Dr. Andrew Kaufman, assistant professor of anesthesiology at the New Jersey Medical School, said doctors still do not have the scientific research on the benefits of medical marijuana and determining what doses are effective is a challenge.
"How do we prescribe for it; one puff, two puffs?" he said. "Who will manage the centers that grow it? I'd like to see standards so if we are writing it, you are getting the same thing every time."
Marijuana is effective as an appetite stimulant, he said, but is not as reliable for pain relief.
"I've had a few patients say they tried it and it didn't do anything for them," he said. "A few others, a smaller subset, tried once in a awhile and it seems to quiet the pain down."
The American Medical Association has urged the federal government to reconsider its classification of marijuana -- now in the most restrictive category with LSD and heroin -- to open the door to more of the research Kaufman is looking for.
As director of University Hospital's Comprehensive Pain Center, Kaufman deals with "people at the end of the road."
"They come to me not because they are starting their journey, but ending their journey," he said. 'I have a patient with MS with burning pain all over her body and I am going to use every tool in my chest. But I have a problem with marijuana because of the way it is delivered; smoking."
Marijuana contains more than 400 different chemicals, including some of the most dangerous ones found in tobacco smoke, according to the U.S. Drug Enforcement Administration. Unlike tobacco, it is smoked without filters.
Now, under the new law, Kaufman might consider prescribing marijuana in conjunction with other medicines. But, he said, morphine works better for overall pain.
But he still has reservations.
"There are marijuana addicts," he said. "Now we get into another can of worms with addiction and behavioral problems."
But that hasn't been the case with O'Brien.
"I go right to the edge, just before the high, and the pain in my hands and feet stops," he said. "I could get stoned out of mind but I don't want to. I want pain relief; that's what I'm after."
He said he worries more about addiction with the strong pain medications his doctors have prescribed over the years and which "cloud" his thinking.
One doctor put him on an antidepressant. "I am nowhere near depressed," he said. "The last doctor wanted to put needles in my fingers and toes with an experimental drug, which was absolutely ridiculous."
O'Brien, the father of a 22-year-old son and a 21-year-old daughter, has been honest with his children about his illegal marijuana use.
"As far as teaching our children, we can't lie to them," he said. "They are smart enough to see the difference when they see the pain I go through daily. This is my only option."