"We found support for the synergistic effect on an individual level; people who tend to use more marijuana also tend to use more opioids," said Scott Novak, PhD, senior research scientist at RTI International, a leading research institute in Research Triangle Park, North Carolina.
What's worrisome is that the positive association between cannabis and opioid use is growing stronger. The study found that over time, people were using these drugs more often; there were more opioids days and more cannabis days.