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.
Read this if you are interested in whether combining cannabis with opioid pain medications is a viable strategy for pain management.
THC enhanced pain relief tolerance 3-10x but did not worsen opioid withdrawal
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Controlled animal study with systematic dose-response measurements, but very small sample size and uncertain clinical translation.
When this study was published
Published in 2016. Interest in opioid-cannabinoid combinations has grown substantially since, with more human data becoming available.
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.
Questions still open
- 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?
Common questions
Does THC help opioid pain medications work better?
Does combining THC with opioids make withdrawal worse?
Read the original research
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
Citation
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
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