Although medicine has advanced far enough to treat basic headaches, strained muscles and the agony of having a cavity filled, inflammatory pain—the kind that results from osteoarthritis, bone cancer and back injuries—has proved to be a far more elusive target. Current remedies, including morphine and other opiates, flood all the nerves of the body, causing dangerous side effects. More localized remedies, such as steroid injections, wear off over time. Recently researchers have begun working with a toxin found in a Moroccan cactuslike plant that may be able to deliver permanent, local pain relief with a single injection.

The compound, called resiniferatoxin (RTX), works by destroying the neurons specifically responsible for inflammatory pain. These neurons extend from the body's periphery (including the skin and internal organs) to the spinal cord, carrying pain signals along their axons. The signals eventually travel up to the brain. When injected directly into spinal fluid, RTX homes in on and kills only those neurons that produce a protein called TRPV1, which transmits the sensation of noxious heat and inflammation. It does not harm normal tissue and other pain-sensing nerves, such as those that produce the feeling of pinpricks or pinches.

RTX has been tested in pet dogs that suffer from debilitating pain, and the studies have shown promising results. Unlike rodents, dogs experience pain much the way people do. “And they have personalities,” says Andrew Mannes, chief of the department of perioperative medicine at the National Institutes of Health. “We can get insight into their psyches that we can't with rats.”

The NIH is now running a trial of RTX in people with advanced cancer. Although Mannes and his colleagues cannot predict how soon they will have data, pain experts are watching the trial with interest. David Maine, director of the Center for Interventional Pain Medicine at Mercy Medical Center in Baltimore, says there are other ways to kill pain fibers, such as using alcohol to destroy nerves, but they sometimes cause the pain to come roaring back, far worse than before. “When you can streamline where a drug acts and avoid consequences outside of that, you potentially have a winner,” Maine says.