It’s well known that a placebo can relieve pain, but how such non-active ingredients can have such a positive effect has long stumped scientists. Now, new researchers suggests that placebos may help ease a patient’s pain by activating cannabinoid receptors, which are also targeted by marijuana, according to a study published October 2(Sunday) in Nature Medicine.

Researchers can give subjects an opioid prior to wrapping their arm with a painfully tight tourniquet. Thereafter, volunteers can tolerate pain longer when given a placebo instead of drugs on follow-up days, presumably because they’ve been primed to expect pain relief from the drug. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as iburprofen can also be used as a primer to induce this placebo-pain relief effect.

In the new study, researchers gave volunteers an NSAID prior to a painful stimulus, followed by rimbonant, which blocks the activation of a cannabinoid receptor, on...

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