Some of the more effective and commonly used medications for chronic pain are drugs that were developed to control other conditions. Antidepressants are a mainstay in the treatment of many chronic pain conditions — even when depression isn't a factor.

Amitriptyline is the antidepressant most commonly prescribed for pain, and it's the one that has been studied most thoroughly. Other tricyclic antidepressants used for pain include:

• Imipramine (Tofranil)
• Nortriptyline (Pamelor)
• Desipramine (Norpramin)

Tricyclic antidepressants seem to work best for the burning or searing pain common after nerve damage, which sometimes occurs with diabetes, shingles or strokes. These drugs are also effective in some people for fibromyalgia, or as a preventative for migraines.

The painkilling mechanism of these drugs is still not fully understood. Tricyclic antidepressants may increase neurotransmitters in the spinal cord that reduce pain signals. But they don't work immediately. You may have to take a tricyclic antidepressant for several weeks before it starts reducing your pain.

Tricyclic antidepressants don't cause dependence or addiction, and they're safe to take for long periods of time. But they can make you drowsy. To manage this side effect, take your tricyclic in the evening, just before bed. In addition, these drugs may cause dry mouth, constipation, weight gain, difficulty with urination and changes in blood pressure. If you have heart disease, these medications may not be a good choice. Discuss it with your doctor.

To reduce or prevent side effects, your doctor will likely start you at a low dose and slowly increase the amount. Most people are able to take tricyclic antidepressants, particularly in low doses, with only mild side effects. The doses that are effective for pain are typically lower than the doses used for depression.

Two newer forms of antidepressants — selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) — have fewer side effects than tricyclic antidepressants and are sometimes used to treat chronic pain.

SSRIs include such drugs as paroxetine (Paxil) and fluoxetine (Prozac). People who have chronic pain may feel better while taking SSRIs, but this effect is believed to result more from the easing of accompanying depressive symptoms rather than from actual pain relief. Drugs such as venlafaxine (Effexor) and duloxetine (Cymbalta) are SNRIs, which appear to be more effective than SSRIs at pain control — particularly pain caused by damaged nerves.