The Centre for Addiction and Mental Health Monitor eReport found, on average, marijuana users in 2005 were 31 years old compared to their 26-year-old counterparts in 1977.

However, Dr. Jurgen Rehm, the center's senior scientist and one of the study's lead investigators, said marijuana use in Ontario among adults is generally infrequent -- less than once a month. Two percent of marijuana users report hazardous levels of use.

Past-year smoking rates are the lowest on record, declining from 28 percent in 1996 to 20 percent in 2005, the report said.

In addition, rates of drinking and driving have steadily declined from 13 percent in 1996 to 6 percent in 2005 -- the lowest on record -- but the rates of weekly binge drinking among men and among young adults still remain at elevated levels.

"Substance use, mainly tobacco and alcohol use, costs Canada almost $9 billion in healthcare costs alone each year," Rehm said.