For the past century, scientists have struggled to create an opiate that stops pain without creating tolerance and addiction. Today, genetic engineering may provide some clues.

Recent experiments with rats suggest that, at least at the molecular level, tolerance to morphine can indeed be separated from the physical dependence on it. Specifically, tolerance is the need for larger and larger doses in order to maintain the effect of an initial dose. Dependence refers to the appearance of withdrawal symptoms once the drug is stopped. For years, tolerance and dependence have been thought to go hand-in-hand. Yet, rats missing a particular protein do not require increased doses of morphine to maintain its painkilling effect, but do develop withdrawal when chronic dosing is stopped. The National Institute on Drug Abuse says these experiments lend more hope for the eventual design of addiction-free pain medicines.

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