We and our collaborative researchers at Fukuoka University found that among the vasopressin receptors, the V1b receptor was involved in the development of tolerance to narcotic analgesics, and antagonists of the V1b receptor markedly suppressed the development of the tolerance to narcotic analgesics.
Vasopressin 1b antagonists can be potentially used for suppressing development of tolerance to analgesic effect of a narcotic analgesic, such as morphine. Vasopressin 1b antagonists may be used in combination with narcotic analgesics for the purpose of reducing or preventing the development of tolerance to analgesic effect of narcotic analgesic.

Example:
The effect on development of tolerance to morphine-induced analgesic effect was examined by using an antagonist selective to the Vasopressin 1b receptor, (2S,4R)-1-(5-chloro-1(2,4-dimethoxyphenyl)sulfonyl)-3-(2-methoxy-phenyl)-2 oxo-2,3-dihydro-1H-indole-3-yl)-4-hydroxy-N,N-dimethyl-2-pyrrolidine carboxamide (SSR149415). In combination with morphine (10 mg/kg, subcutaneous), ddy mice were given with a solvent or V1b antagonist twice a day for 4days. These analgesic effect was determined by tail-flick test.
The result showed that by intracerebroventricular administration of the V1b receptor antagonist, the development of tolerance to morphine was successfully suppressed without any effect on the acute morphine-induced analgesic effect.