In 1969, Carol McDonald was 28, married and the mother of two young children, out for an evening of fun with a couple who smoked marijuana. By the end of the evening she was on her way to a 19-year addiction.
"Within a few months, I was smoking every day," said McDonald, a retired bookkeeper, now 69. "I had to smoke before going to work. If something was upsetting, I smoked over it. If there was a celebration, I smoked over it."
People like McDonald may be largely overlooked in the statewide debate over legalizing marijuana. The drug has a benign reputation: Many baby boomers smoked and emerged unscathed, and medical marijuana facilities with their friendly images of seven-fingered leaves have popped up all over Los Angeles.
That might be why Proposition 19, the Nov. 2 ballot measure that would legalize marijuana and regulate it similarly to alcohol, has generated scores of reports and debates regarding the potential effect on business revenue, tax dollars and law enforcement but scant discussion on the potential fallout on people's health.
In California, addiction counselors are split on the legalization issue largely because of their long-standing support of treatment over jail and legal penalties for marijuana addicts. Yet nationally, public health experts mostly are against legalization. They say it will increase the number of people who become addicted to the drug, contribute to more automobile accidents and erode school performance.
"It's bizarre to me when people say, 'Make marijuana legal, and we'll have no problems with it,' " said Keith Humphreys, a professor of psychiatry and behavioral science at Stanford University who recently served as a White House senior advisor on the nation's drug control policy.
Because the science of marijuana's health effects is in many cases unclear, experts on each side of the legalization debate can point to scientific studies that support their own position.
They do agree that marijuana should be avoided during pregnancy and that it is harmful for people with mental illness or who are at risk for developing a serious mental illness, such as schizophrenia.
And they agree, too, on some basic statistics: Marijuana is addictive for about 9% of adults who use it (compared with about 15% who use alcohol and 15% who use cocaine), according to federal data. Because it is the most widely used illegal substance in the country, marijuana dependence is more common than addiction to either cocaine or heroin despite its lower addiction potential.
"We generally think the problems with marijuana aren't as serious as the problems you tend to see with cocaine or heroin," said Alan J. Budney, a leading researcher on marijuana at the Center for Addiction Research at the University of Arkansas for Medical Sciences who opposes legalization. "But they are still pretty substantial."
The science of marijuana becomes murky when one steps beyond addiction statistics to examine effects on health.
A series of studies conducted by the National Highway Traffic Safety Administration published in 1998 found that the effects of marijuana alone on driving were small or moderate, but severe when combined with alcohol.
But other studies show little impairment from a moderate dose: A 2004 study in the journal Accident, Analysis and Prevention found no increased risk of motor vehicle accidents causing traumatic injury among drivers using marijuana.
"Even after smoking, there aren't any real deficits in driving ability that we can detect in the laboratory," said Mitch Earleywine, an associate professor of psychology at the State University of New York at Albany who serves as an advisory board member at the National Organization for the Reform of Marijuana Laws.
The data on lung damage and smoking-related cancers are similarly mixed, in part because a large portion of heavy marijuana users also smoke tobacco, which muddies the picture of marijuana's effects. And though experts tend to agree that smoking marijuana causes short-term memory loss, they disagree widely on the overall cognitive effects of the drug.
Several studies have also dismissed the fear that marijuana is a "gateway" drug that will lead children and adolescents to experiment with harder illicit drugs — although numerous studies suggest that the earlier in life someone uses marijuana, the riskier it becomes. Among 14- and 15-year-olds who start to smoke, 17% will be dependent within two years, said Dr. Tim Cermak, an addiction psychiatrist and president of the California Society of Addiction Medicine.
The effect on school performance and learning could be significant if more minors use the drug, Cermak added. "Marijuana is not devastating in the same way alcohol is," he said. "But to an adolescent, it can impact their life permanently. When you take a vacation from development in school for five years, you just don't get to the same endpoint that was available to you earlier in life."