BACKGROUND AND PURPOSE: Although morphine is a very effective analgesic, its narrow therapeutic index and severe side effects limit its therapeutic use. Previous studies indicated that the pharmacological responses of opioids are modulated by genetic and pharmacological invalidation of tachykinin receptors. Here we address the role of substance P and neurokinin A, which are both encoded by the tachykinin 1 (tac1) gene, as modulators of opioid effects. EXPERIMENTAL APPROACH: The analgesic and side effect potential of morphine was compared between wild-type and tac1 null mutant mice. KEY RESULTS: Morphine was a more potent analgesic in tac1 null mutant mice, that is, in the absence of substance P/neurokinin A signalling. Interestingly, the most serious side effect of acute morphine, that is respiratory depression, was reduced in tac1(-/-) animals. Comparing the addictive potential of morphine in wild-type and knockout animals we found that morphine preference was similar between the genotypes. However, the aversive effect of withdrawal precipitated by naloxone in morphine-dependent animals was significantly reduced in tac1 knockout mice. Behavioural sensitization, the underlying mechanism of addiction, was also significantly lower in tac1(-/-) mice. CONCLUSION AND IMPLICATIONS: The analgesic potential of morphine was increased in tac1 knockout mice. In contrast, both the ventilatory suppressing effect and the addictive potential of morphine were reduced. These results suggest that reducing activity of the tachykinin system may be a possible strategy to improve the pharmacological potential of morphine.