He has served on the Expert Committee on Drug Dependence for the World Health Organization and as an editorial consultant for the medical journal, Lancet, and currently is Professorial Research Fellow at the University of Queensland in Australia.
"The epidemiological literature in the past 20 years shows that cannabis use increases the risk of accidents and can produce dependence, and that there are consistent associations between regular cannabis use and poor psychosocial outcomes and mental health in adulthood," Hall wrote in the October edition of the scientific journal Addiction.
Although he found no chance of death by overdose, he uncovered alarming acute and chronic conditions related to marijuana smoking.
Importantly, he found that ten percent of all pot smokers and 1 in 6 of teen users develop dependence syndrome.
Moreover, teen pot smokers were more likely to use other illicit drugs and doubled their risks of dropping out of school "and of cognitive impairment and psychoses in adulthood," he wrote.
These negative outcomes for youth are exacerbated by the fact that marijuana use among high school students has increased from 31.3 percent in 1991 to 40.7 percent in 2013, according to the U.S. Centers for Disease Control and Prevention. Also, the Denver Post reported last year, following Colorado's 2012 vote to legalize recreational use, school officials announced a surge in marijuana use among middle school students.
Hall also discovered that the seeming innocuous symptoms of relatively short-term impairment like delayed reaction time and decreased motor skills had devastating effects when pot smokers attempted to drive -- it doubled their risk of a car crash. Overall, cannabis users had higher rates of hospitalization for injury from all causes than former pot users or non-users.
Other findings show regular marijuana use causes:
-- double the risk of suicide among regular users
-- 3 to 4 times the risk for myocardial infarction among middle-aged and older adults
-- 2-3 fold increase in risk for testicular cancers for men
Hall pointed out marijuana potency has increased measurably since 1980, with THC content rising from less than 2 percent to more than 8.5 percent in 2006.
His analysis only looked at studies involving regular, typically daily, cannabis smoking.
Hall is a member of the International Narcotics Control Board, an independent, quasi-judicial expert body established by the United Nations as the monitoring body for the implementation of its international drug control conventions. His findings were first presented at the Through the Maze: Cannabis and Health International Drug Policy Symposium in Auckland, Zealand in November 2013.