Adding THC to Morphine Increases Pain Tolerance Buildup but Does Not Worsen Withdrawal in Monkeys
Rhesus monkeys receiving both morphine and THC developed greater pain relief tolerance than those on morphine alone, but THC did not make opioid withdrawal worse when treatment stopped.
Quick Facts
What This Study Found
Combining opioids with cannabinoids is proposed as a way to enhance pain relief while potentially reducing opioid doses. This study tested what happens with prolonged combined treatment in monkeys.
Before treatment, morphine and THC both effectively eliminated pain responses. However, during twice-daily combined treatment, tolerance developed faster and more extensively than with morphine alone. Pain-relieving doses needed to increase 3 to 10 times above pre-treatment levels.
The key finding was about withdrawal. When treatment stopped, monkeys showed increased heart rate and observable withdrawal signs, but these were generally similar whether they had received morphine alone or morphine plus THC. So while THC enhanced morphine's pain-relieving effects and accelerated tolerance, it did not worsen the physical dependence picture.
Key Numbers
Tolerance produced 3 to 10-fold rightward shifts in pain relief dose-response curves. Greater shifts occurred with the morphine/THC combination than morphine alone. Withdrawal signs (heart rate increase, behavioral signs) were generally similar between treatment groups.
How They Did This
Four rhesus monkeys received twice-daily injections of morphine (3.2 mg/kg) alone or combined with THC (1 mg/kg). Pain relief was measured using warm water tail withdrawal latency. Researchers tracked antinociceptive effects before, during, and after treatment, and monitored withdrawal signs when treatment was discontinued.
Why This Research Matters
The opioid epidemic has driven interest in combining opioids with cannabinoids to reduce opioid doses. This study provides mixed news: while the combination enhances acute pain relief, it also accelerates tolerance, which could undermine the dose-reduction strategy. The reassuring finding is that THC does not appear to worsen opioid dependence.
The Bigger Picture
This study complicates the narrative around opioid-cannabinoid combinations for pain management. While acute enhancement is real, faster tolerance development could mean patients need dose escalations sooner. The independence of the dependence and tolerance findings suggests these processes are mediated by different mechanisms.
What This Study Doesn't Tell Us
Small sample size of four monkeys. Animal pain models may not translate directly to human chronic pain. The study used specific doses that may not reflect clinical ratios. The THC dose used was relatively high.
Questions This Raises
- ?Would lower THC doses still enhance morphine analgesia without accelerating tolerance?
- ?Does the tolerance finding apply to human chronic pain patients?
- ?Would CBD instead of THC avoid the tolerance acceleration?
Trust & Context
- Key Stat:
- THC enhanced pain relief tolerance 3-10x but did not worsen opioid withdrawal
- Evidence Grade:
- Controlled animal study with systematic dose-response measurements, but very small sample size and uncertain clinical translation.
- Study Age:
- Published in 2016. Interest in opioid-cannabinoid combinations has grown substantially since, with more human data becoming available.
- Original Title:
- Combined Treatment with Morphine and Δ9-Tetrahydrocannabinol in Rhesus Monkeys: Antinociceptive Tolerance and Withdrawal.
- Published In:
- The Journal of pharmacology and experimental therapeutics, 357(2), 357-66 (2016)
- Authors:
- Gerak, L R, France, C P
- Database ID:
- RTHC-01162
Evidence Hierarchy
Tests effects in animals (usually mice or rats), not humans.
What do these levels mean? →Frequently Asked Questions
Does THC help opioid pain medications work better?
Yes, in the short term. THC enhanced morphine's pain-relieving effects in monkeys. However, the combination also led to faster tolerance development, meaning higher doses were needed sooner.
Does combining THC with opioids make withdrawal worse?
No. In this monkey study, withdrawal signs were generally similar whether morphine was given alone or with THC, suggesting THC does not worsen opioid physical dependence.
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Cite This Study
https://rethinkthc.com/research/RTHC-01162APA
Gerak, L R; France, C P. (2016). Combined Treatment with Morphine and Δ9-Tetrahydrocannabinol in Rhesus Monkeys: Antinociceptive Tolerance and Withdrawal.. The Journal of pharmacology and experimental therapeutics, 357(2), 357-66. https://doi.org/10.1124/jpet.115.231381
MLA
Gerak, L R, et al. "Combined Treatment with Morphine and Δ9-Tetrahydrocannabinol in Rhesus Monkeys: Antinociceptive Tolerance and Withdrawal.." The Journal of pharmacology and experimental therapeutics, 2016. https://doi.org/10.1124/jpet.115.231381
RethinkTHC
RethinkTHC Research Database. "Combined Treatment with Morphine and Δ9-Tetrahydrocannabinol..." RTHC-01162. Retrieved from https://rethinkthc.com/research/gerak-2016-combined-treatment-with-morphine
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.