"These results indicate that some marijuana users experience withdrawal effects when they try to quit, and that these effects should be considered by clinicians treating people with problems related to heavy marijuana use," Vandrey says in a news release.
Vandrey's team studied 12 people who were heavy smokers of both tobacco and marijuana.
For the study's sake, participants stopped smoking marijuana -- but kept smoking tobacco -- for five days. Then they were free to smoke both substances for nine days.
Next, participants stopped smoking tobacco and kept smoking marijuana for five days. Then they smoked as they pleased for nine days, followed by five days without marijuana or tobacco.
Participants rated their withdrawal symptoms and took drug tests every day throughout the study. They noted similar symptoms when they stopped marijuana, tobacco, or both substances.
The three nonsmoking periods -- no marijuana, no tobacco, and no marijuana or tobacco -- were equally difficult, according to the study.
Some participants -- but not all -- found it easier to stop smoking marijuana and tobacco at the same time, instead of halting only one of those drugs. Smoking just one substance may have made them want the other one, too, Vandrey's team notes.