Do Medical Marijuana Laws Reduce Opioid Use After Cancer Surgery? | Research

The takeaway

After states legalized medical marijuana, cancer surgery patients were less likely to receive strong opioid prescriptions—but only once dispensaries actually opened.

Oncologists considering cannabis as adjunctive pain management, health policy researchers studying marijuana law impacts, and cancer patients navigating post-surgical pain options.

What the researchers found

Using insurance claims data from over 34,000 cancer surgery patients across 27 states, this study examined whether medical marijuana legalization (MML) affected opioid prescribing patterns in the six months after diagnosis.

The researchers distinguished between two phases of legalization: the law passing (MML without dispensaries) and dispensaries actually opening (MML with dispensaries). This distinction proved critical. Simply passing a medical marijuana law had no measurable effect on opioid prescribing. But once dispensaries opened and patients could actually access cannabis, prescriptions for strong short-acting opioids (oxycodone, hydrocodone, hydromorphone, morphine) decreased.

The study used a difference-in-differences approach—comparing changes over time in states that legalized to changes in states that didn't—which helps account for national trends in opioid prescribing that affected all states.

The findings were specific to strong opioids. Weak opioid prescriptions (tramadol, codeine), total morphine milligram equivalents among those who did receive opioids, and emergency department visits didn't change significantly. This suggests patients may have been substituting cannabis for some strong opioid use rather than reducing all pain management.

Why it matters

Cancer patients routinely receive opioids after surgery, and the opioid crisis has made clinicians and patients wary of these prescriptions. If medical cannabis can substitute for some strong opioid use after cancer surgery, that could reduce opioid-related risks—dependence, respiratory depression, constipation—without necessarily improving or worsening pain control. The dispensary access finding also matters for policy: laws on paper don't change behavior; actual access does.

The numbers in context

N = 34,911 cancer surgery patients. 24,592 breast, 8,510 colorectal, 1,809 lung cancer. MML with dispensaries associated with reduced strong opioid prescriptions. MML without dispensaries: no significant effect. Weak opioids and total MME: no significant change.

How the study worked

Cross-sectional study using a difference-in-differences design with 2016–2022 private insurance claims data. N = 34,911 patients aged 18–64 undergoing resection surgery for breast (24,592), colorectal (8,510), or lung (1,809) cancer across 27 states without MML as of 2016. MML policies classified as: no MML, MML without dispensaries, and MML with dispensaries. Outcomes measured during 6 months post-diagnosis.

Who was studied

N=34,911 patients aged 18-64, undergoing cancer resection surgery in 27 US states without medical marijuana legalization as of 2016.

What this study cannot tell us

Private insurance claims only (excludes Medicare, Medicaid, uninsured). Ages 18–64, so doesn't capture older cancer patients who make up the majority. Claims data can't confirm whether patients actually used cannabis. The difference-in-differences design assumes parallel trends between legalizing and non-legalizing states, which may not hold perfectly. Cannot determine whether the opioid reduction improved or worsened pain outcomes.

How to read the evidence

Large observational study with a difference-in-differences design that provides stronger causal inference than simple cross-sectional analysis, though it can't prove causation.

When this study was published

Published in 2025 with data from 2016–2022, capturing the period of rapid state-level medical marijuana expansion.

The bigger picture

This dovetails with RTHC-00158's finding that THC:CBD oil specifically improved pain in cancer patients but not overall symptoms. Together, they paint a picture of cannabis as a potential pain-specific tool in cancer care. The dispensary access finding also parallels broader health policy research showing that health laws only work when infrastructure supports them—a pattern seen in everything from ACA insurance mandates to naloxone access laws.

Replication

Not stated in abstract.

Funding

Not reported in abstract.

Conflicts of interest

Not reported in abstract.

Questions still open

Did the reduction in strong opioid prescriptions translate to less opioid-related harm (overdose, dependence)? Were patients actually using medical cannabis, or did the law change prescribing norms independently? Would results be similar for Medicare patients or in fully recreational legalization states?

Read the original research

Medical marijuana policies, opioid prescriptions, and adverse events among patients undergoing cancer resection surgery.

Cancer, 131(19), e70107

Cancer is a well-respected journal focusing on oncology research.

View on PubMed ↗Publisher’s paper ↗

Citation

Hu, Ju-Chen; Karan, Kenneth; Zhang, Hao; Portenoy, Russell; Rosa, William E; Zhang, Yiye; Reid, M Carrington; Tamimi, Rulla M; Zhang, Fang; Bruera, Eduardo; Paice, Judith A; Bao, Yuhua. (2025). Medical marijuana policies, opioid prescriptions, and adverse events among patients undergoing cancer resection surgery.. Cancer, 131(19), e70107. https://doi.org/10.1002/cncr.70107