Daily marijuana use has been on the rise over the past decade.

Now, a new study by researchers at Columbia University's Mailman School of Public Health and the Graduate School of Public Health and Health Policy, City University of New York, found that cigarette smokers are 10 times more likely to use marijuana on a daily basis.

Marijuana use occurred nearly exclusively among current cigarette smokers--daily or non-daily smokers--compared with former smokers and those who have never smoked.

However, even among non-smokers, daily marijuana use is increasing, particularly among youth and female cigarette smokers.

The findings are published online in the American Journal of Public Health.

"While we found that daily cannabis use and cigarette smoking were strongly linked among all subgroups, the most striking finding in daily cannabis use was among youths aged 12 to 17 years," said Renee Goodwin, PhD, Department of Epidemiology, Mailman School of Public Health, and principal investigator.

"Nearly one-third of youth who smoke cigarettes reported using cannabis every day. In contrast, less than 1 percent of youth who did not use cigarettes reported daily cannabis use. We are not aware of any previous reports illustrating that daily cannabis use in youths occurs nearly exclusively among those who smoke cigarettes."

The researchers analysed data from 725,010 individuals ages 12 and older in the National Survey on Drug Use and Health for 2002 to 2014 to determine differences in the prevalence of daily cannabis use.

They found that an increase in daily cannabis use was significantly higher among non-daily cigarette smokers than among daily smokers and among former smokers than among never smokers.

Individuals who reported smoking fewer than 100 cigarettes in their lifetime were classified as never smokers; those who reported smoking 100 cigarettes or more in their lifetime and at least 1 cigarette within the past 30 days were considered to be current smokers.

Those who smoked 1 to 29 days of the past 30 days were categorised as current non-daily smokers, and those who smoked all 30 of the past 30 days as current daily smokers.

Individuals who reported smoking 100 cigarettes in their lifetime and no cigarettes in the past 30 days were grouped as former smokers.

Daily cannabis use increased since 2002 among both non-daily smokers (8 percent in 2014 compared with 3 percent in 2002) and daily smokers (9 percent in 2014 versus 5 percent in 2002).

The increase in daily cannabis use was faster among non-daily cigarette smokers relative to daily cigarette smokers.

Daily cannabis use increased most rapidly among former cigarette smokers (2.80 percent in 2014 versus 0.98 percent in 2002).

"Using marijuana as an alternative substance is viewed as less addictive, less harmful, and carrying less stigma than cigarettes," said Goodwin.

"Some clinical data suggest that marijuana lessens the experience of nicotine withdrawal, and people who quit smoking cigarettes might substitute marijuana to lessen their withdrawal symptoms."

The fastest rates of increase of cannabis use overall were among those aged 26 years and older versus aged 12 to 17 years and 18 to 25 years.

However, cigarette smokers aged 12 to 17 were 50 times more likely to be daily cannabis users than youth who do not use cigarettes.

In 2014, 28 percent of daily cigarette smokers and 13 percent of non-daily cigarette smokers aged 12 to 17 used cannabis daily, suggesting that 40 percent of 12 to 17 year olds who smoke cigarettes used cannabis daily in 2014.

Among female cigarette smokers, 4 percent used cannabis daily.

Cigarette smoking remains the leading preventable cause of disease and premature mortality in the United States.

There have been substantial declines in smoking prevalence over the past half century in the United States, although the rate of this decline has decelerated in recent years among various groups.

"It is conceivable that this stunted decline in cigarette use is owing, in part, to the substantial increase in daily cannabis use among smokers," observed Goodwin.

"Understanding the degree to which daily cannabis use may be common among cigarette smokers is critical because previous findings suggest that any past month cannabis use is associated with smoking persistence and relapse," noted Goodwin.

Source: Columbia University Mailman School of Public Health