Researchers at the University of Michigan will try to document the impact of medical marijuana, with a $2.2 million grant from the National Institute on Drug Abuse. The study will include 800 Michigan patients who want to obtain state certification for medical marijuana to treat their pain. They will be tracked for two years.

Currently the state has more than 135,000 patients who have been approved to use medical marijuana, which was approved by voters in 2008.

Most previous studies on medical marijuana have been conducted in the lab, where patients are given different amounts of marijuana or a placebo, and then report their level of pain. These studies tend to last only hours or days. This study will look at medical marijuana’s long-term effect on patients’ lives in a variety of settings, the Detroit Free Press reports.

The new study will measure patients’ self-reported pain levels, in addition to other data including drug screens for other substances, everyday functioning and use of health care services, according to study leader Mark Ilgen, PhD.

Participants will be asked to join the study when they are at their first doctor’s appointment, as they try to obtain medical marijuana certification. They will answer questions at the beginning of the study, and again every six months over two years.

“With the ongoing policy debate and the growing popularity of medical marijuana programs in the United States, it is essential to understand the ramifications of medical marijuana use for individuals who seek access to it,” Ilgen said in a news release. “We hope that this study can help inform the debate.”

Study co-investigator Frederic Blow, PhD, says that while marijuana is the most frequently used drug in the United States, and has been legalized for medical use in many ways, there is very little understanding of how people using medical marijuana do over time.