Pain. It stabs. It burns. It aches. It throbs. It gnaws at you. It knocks you for a loop. But, sooner or later, it goes away.

Unless it doesn’t.

That’s a nightmare come true for millions of Americans who spend every day in a world of hurt. And the problem will get only bigger. “As our demographics change, and we live longer, more people will experience chronic pain,” says Dr. Lynn Webster, medical director of the Lifetree Clinical Research and Pain Clinic in Salt Lake City.

Pain is usually a symptom of something else — a scraped knee, a broken arm, appendicitis. Treating the pain makes the patient less miserable, but it’s just a stopgap measure until the underlying problem is fixed and the pain goes away — the scrape heals, the bone knits back together, the appendix is removed.

With chronic pain, however, the underlying problem that started it has usually (though not always) been fixed and yet the patient is still hurting. A malfunctioning nervous system has started manufacturing pain. The pain is no longer simply a symptom. It has become a problem in its own right.

No one knows a sure-fire way to avoid chronic pain. Still, you can improve your chances by avoiding the temptation to simply tough it out when you get injured. “Luckily, if treated adequately, pain goes away in a majority of patients,” says Dr. Talal Khan, a specialist in anesthesiology, pain management and pain medicine at the University of Kansas Hospital in Kansas City. “But once chronic pain develops, it can be very hard to cure.”

And then it can be devastating. “Patients are often disabled,” Webster says. “They live with a tremendous amount of pain even with top-of-the-line treatment.”

By definition, pain is a sensation, generally associated with tissue damage, that is experienced as unpleasant. Pain is said to be chronic if it occurs every day, for most of the day, for three months. (Some set the threshold sooner, some later.) Though precise numbers are hard to find, it’s estimated that one-third or more of Americans will have a bout of chronic pain during their lifetime.

Sometimes the pain persists because the injury persists — for example, in patients with arthritis or cancer. Often, though, the pain-causing injury heals, but the pain just doesn’t get the memo.

Still, even while defined by its persistence, chronic pain isn’t just a longer-lasting version of acute pain.

Acute pain is actually very useful. If you stub your toe, the resulting pain sends you two messages: “Watch where you’re going!” and “Don’t plan on running any marathons until your toe feels better!” In general, it teaches you to avoid doing again whatever you did that caused it in the first place. And it warns you to give your injury time to heal.

Chronic pain, on the other hand, has outlived its usefulness. “It sets your body on high alert,” Khan says, “even though the threat may not be there.”

The physiological processes involved in acute and chronic pain are also different. You burn your finger. You stub your toe. You trip over your dog’s leash and fall flat on your face. The acute pain you feel results from nature’s own instant-messaging service: Your injury is converted into nerve impulses that zip off to your spinal cord and then on to your brain, the place where pain is actually perceived. The pain messages bombard your brain for a while, tapering off as your injury heals, and stopping altogether when you’re all better.

In chronic pain, this standard operating procedure is subverted — the brain, in a sense, is acting too smart for its own good.

“The transition to chronic pain is similar to learning,” says Dr. Richard Lipton, a neurologist and director of the Montefiore Headache Center in the Bronx, N.Y. It’s great if the brain learns to ride a bike, or hit a tennis ball or solve calculus problems. Not so great if it learns to make you hurt all the time.

Unfortunately, Lipton says, “the brain gets better at whatever it practices. If it practices being in pain, it gets better at being in pain.”

The process involved is called sensitization, and it can occur at two levels. In one of them, nerve endings that pick up pain messages at the injury site and send them to the spinal cord become all fired up about things that wouldn’t faze them ordinarily — just the slightest touch can set them off. And the touch doesn’t even have to be at the injury site. The general vicinity may be close enough.

Less often, and usually later, nerves in the central nervous system — in the spinal cord or in the brain itself — get into such a habit of sounding the pain alarm that if they no longer receive pain messages from the injury site, they start rumors all on their own.

The chronic pain process can also involve the ability of the nervous system to reorganize itself and grow new connections. This ability is vitally important in helping stroke patients recover functions and sensations they have lost. “But it perpetuates, augments, aggravates and increases pain,” Webster says. “Within hours of an acute injury, we see small little nerve fibers sprouting. Sometimes they not only transmit pain and increase sensitivity, but they also begin to produce their own pain.”

Once this “rewiring” occurs, it’s very difficult to reverse, Webster says. And even if it is reversed, it’s liable to recur. It’s similar to addiction in that way: Though alcoholics can be detoxed, they’re still alcoholics.

It’s clear from all this that the best way to avoid developing chronic pain is to avoid acute pain in the first place, Khan says: “Try to stay in good health, have a good diet, be careful lifting heavy objects.” And since chronic pain can develop after surgery — which may lead to changes in the nervous system within hours, courtesy of those pesky little nerve sprouts — some doctors try to take preventive measures before they even operate, for example administering anti-inflammatories orally or injecting local anesthetics around targeted nerves.

Still, everyone will experience acute pain at times, and some people will go on to develop chronic pain. Although it’s not clear exactly why some do and some don’t, Khan cites three probable factors: genetic differences; cultural, societal and personal differences relating to pain experiences; and the severity of the initial acute pain.

Just as there’s no one impetus for chronic pain, there’s no one way to treat it, so pain specialists advise a multidisciplinary approach.

“We don’t talk about a cure,” says Marilyn Jacobs, a clinical psychologist and voluntary assistant professor in the departments of anesthesiology and psychiatry at UCLA. “But with standard-of-care pain treatment, the chances of managing it are much higher.”

Such treatment includes drugs, primarily anti-inflammatories and narcotics. These painkillers take a variety of approaches to dialing down pain messages that are sent to the brain. Some interfere with messages sent from the injury site to the spinal cord, others block transmission up the spinal cord, and still others mimic natural painkillers that the brain itself makes and delivers to the injury site.

In rare cases, doctors may use a pain pump — a reservoir of drugs implanted in the abdomen, delivering medication directly to the spine in smaller doses than would be needed orally. Another possibility is a spinal cord stimulator, which sends electrical charges to the spinal area to try to block transmission of pain signals to the brain. But these options are used only in extreme cases, Jacobs says — if all else fails.

Standard-of-care treatment also includes physical and behavioral therapy. Even plain old distraction can help. “Often, pain is worst at night, because you’re inactive, so pain input is unopposed,” Webster says. When you use a different part of your brain on other things — listening to music, talking with friends, petting your dog — the pain input has some healthy competition. There’s only so much the brain can process at once.

But good treatment will also address psychological issues. “We’re hot-wired to experience pain on both a sensory and emotional level,” says Dr. F. Michael Ferrante, professor of anesthesiology and director of the UCLA Pain Management Center. “If you have chronic pain, it’s natural to get depressed.”

Patients often need help coming to terms with social losses they may face — loss of a job, of people they used to be close to and activities they used to enjoy. They also confront a lot of misunderstanding, from friends, relatives, employers and even doctors who may not believe them or take their suffering seriously.

“Pain is an intensely private experience,” Lipton says. “In clinical practice, the key is to listen to what the pain sufferer says.”

In fact, of all the tools at a pain doctor’s disposal, listening may be the most powerful. “The most therapeutic thing I can do is to acknowledge my patients’ pain and believe them,” Webster says. “It’s amazing how many people take a big sigh and begin to cry.”

health@latimes.com