The study does not establish a causal connection between marijuana use and sexual activity, Eisenberg noted. But the results hint at it, he added. “The overall trend we saw applied to people of both sexes and all races, ages, education levels, income groups and religions, every health status, whether they were married or single and whether or not they had kids.”

The study is the first to examine the relationship between marijuana use and frequency of sexual intercourse at the population level in the United States.

“Marijuana use is very common, but its large-scale use and association with sexual frequency hasn’t been studied much in a scientific way,” Eisenberg said.

According to the National Institute on Drug Abuse, more than 20 million adult Americans are current marijuana users. With the drug’s legalization for medical or recreational use in 29 states, that number is climbing. But despite marijuana’s growing status as a recreational drug, its status as a procreational drug remains ambiguous: On one hand, there are reports of erectile dysfunction in heavy users, and rigorous studies have found reduced sperm counts in men who smoke it; on the other hand, experiments conducted in animal models and humans indicate that marijuana stimulates activity in brain regions involved in sexual arousal and activity.

Looking at survey responses

To arrive at an accurate determination of marijuana’s effect on intercourse frequency, Eisenberg and Sun turned to the National Survey of Family Growth, sponsored by the federal Centers for Disease Control and Prevention. The survey, which provides data pertaining to family structures, sexual practices and childbearing, reflects the overall demographic features of the U.S. population. Originally conducted at regular intervals, the survey is now carried out on an annual basis. It explicitly queries respondents on how many times they’ve had intercourse with a member of the opposite sex in the past four weeks, and how frequently they’ve smoked marijuana over the past 12 months.

The investigators compiled answers to those questions for all years since 2002, when the survey first began collecting data on men as well as women. They included data from respondents ages 25-45 and excluded a small percentage (fewer than 3 percent) of respondents who had failed to answer one or more relevant questions.

In all, Eisenberg and Sun obtained data on 28,176 women averaging 29.9 years of age and 22,943 men whose average age was 29.5. They assessed these individuals’ self-reported patterns of marijuana use over the previous year and their self-reported frequency of heterosexual intercourse over the previous four weeks.

Some 24.5 percent of men and 14.5 percent of women in the analysis reported having used marijuana, and there was a positive association between the frequency of marijuana use and the frequency of sexual intercourse. This relationship applied to both sexes: Women denying marijuana use in the past year, for example, had sex on average 6.0 times during the previous four weeks, whereas that number was 7.1 for daily pot users. Among men, the corresponding figure was 5.6 for nonusers and 6.9 for daily users.

In other words, pot users are having about 20 percent more sex than pot abstainers, Eisenberg noted.