THC–Morphine Cross-Tolerance Ran in Different Directions for Temperature and Antinociception in Mice
A 1978 mouse comparison found asymmetric cross-tolerance: prior THC reduced sensitivity to morphine hypothermia, while prior morphine reduced sensitivity to THC antinociception. A general two-way cross-tolerance label would conceal the endpoint distinction.
The drugs were compared under common conditions
Both drugs produced antinociception, hypothermia and increased catecholamine synthesis after subcutaneous administration. Their relative effects differed: morphine was more potent on the antinociceptive measure and increased dopamine synthesis more, while THC was more potent for cooling and increased norepinephrine synthesis more.
The cross-challenges were selective
THC-tolerant animals showed cross-tolerance only to morphine's hypothermic action in the reported comparison. Morphine-tolerant animals showed cross-tolerance only to THC antinociception. The observation did not establish equivalent protection across both drugs and both outcomes.
Antagonism was also incomplete
Naloxone partly antagonized THC-related hypothermia and catecholamine-synthesis increases. That supports investigating opioid-related interactions without establishing that THC's effects are mediated entirely through opioid receptors. Similar outward effects do not require identical primary pharmacology.
The study's strength is a matched species-and-strain comparison with several endpoints. It does not provide a dose conversion between THC and morphine or predict a patient's drug response. Exposure direction and measured function are necessary parts of any cross-tolerance claim.
Related: interpreting cannabis tolerance claims.
Research cited
Bloom AS; Dewey WL. A comparison of some pharmacological actions of morphine and delta9-tetrahydrocannabinol in the mouse. Psychopharmacology (1978). doi:10.1007/BF00426745. Findings discussed from the study abstract.
