And a potential solution to it:<p><a href="https://arstechnica.com/science/2017/03/tweaking-fentanyls-chemical-structure-may-create-safer-opioid/" rel="nofollow">https://arstechnica.com/science/2017/03/tweaking-fentanyls-c...</a><p><i>For their chemical makeover, the researchers noted that when tissue is damaged and hurting, it becomes inflamed and more acidic. The pH drops from approximately 7.4—what’s seen in normal, healthy tissue—to between 5 and 7. Fentanyl can work regardless of the pH, so it’s active throughout the nervous system no matter what. But, if it was altered to only work at the lower pH, then it could target just the pain source at the peripheral nerves, the researchers hypothesized. And with no activity in the central nervous system, it would dodge opioid’s serious side-effects, including addiction and systemic responses that can be lethal during overdoses.</i><p>They're testing a modified version of fentanyl that only works in lower pH, restricting its activity to injured areas of the body, eliminating the risk of overdose and dependence:<p><i>In further experiments, the researchers noted that, unlike fentanyl, high doses of NFEPP weren’t lethal to the rats. And rats on the NFEPP didn’t display impaired motor activity or reward-seeking behavior linked to addiction.</i>