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Gabapentin

Gabapentin (brand name: Neurontin®) is an anticonvulsant medication indicated in the treatment of epilepsy and neuropathic pain. Gabapentin is known for having a relatively mild side-effect profile, and passes through the body unmetabolized. more...

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Gabapentin is similar in structure to the neurotransmitter GABA but is not believed to act on the same brain receptors. Its exact mechanism of action is unknown, but its therapeutic action on neuropathic pain is thought to involve voltage-gated calcium ion channels.

Clinical uses

Gabapentin has also been used in the treatment of bipolar disorder. However, its off-label use for this purpose is increasingly controversial. Some claim gabapentin acts as a mood stabilizer and has the advantage of having fewer side-effects than more conventional bipolar drugs such as lithium and valproic acid. Some small, non-controlled studies in the 1990s, most sponsored by gabapentin's manufacturer, suggested that gabapentin treatment for bipolar disorder may be promising. However, more recently, several larger, controlled, and double-blind studies have found that gabapentin was no more effective than (and in one study, slightly less effective than) placebo, and the manufacturer has even halted its own studies regarding gabapentin and bipolar disorder. Despite this scientific evidence against the efficacy of gabapentin in the treatment of bipolar disorder, many psychiatrists continue to prescribe it for this purpose.

Gabapentin has also been used off label in the treatment of anxiety disorders such as social anxiety disorder and obsessive-compulsive disorder, in treatment-resistant depression, and for insomnia. Gabapentin may be effective in reducing pain and spasticity in multiple sclerosis.

In addition to its use in mood disorders, gabapentin was approved by the FDA for treating postherpetic neuralgia (neuropathic pain following shingles).

Marketing of gabapentin

Gabapentin is best known under the brand name Neurontin manufactured by Pfizer . In October 2004, FDA approved a generic equivalent to Neurontin made by Israeli firm Teva. Neurontin is one of Pfizer’s best selling drugs, and was one of the 50 most prescribed drugs in the United States in 2003. However, in recent years Pfizer has come under heavy criticism for its marketing of Neurontin, facing allegations that behind the scenes Pfizer marketed the drug for at least a dozen supposed uses for which the drug had not been FDA approved. By some estimates, so-called off-label prescriptions account for roughly 90% of Neurontin sales. While off-label prescriptions are common for a number of drugs and are perfectly legal (if not always appropriate), marketing for off-label uses of a drug is strictly illegal. In 2004, Pfizer agreed to pay a $430 million settlement for the illegal marketing of Neurontin for off-label purposes, and further legal action is pending.

Pfizer has developed a successor to gabapentin, called pregabalin (being marketed as Lyrica®). Structurally related to gabapentin, Pregabalin is effective for neuropathy pain assciated with diabetes and shingles, and for the treatment of epilepsy and seizures.

Read more at Wikipedia.org


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Gabapentin plus morphine for the treatment of neuralgia
From American Family Physician, 8/15/05 by Mark Ebell

Clinical Question: Is the combination of gabapentin (Neurontin) and morphine more effective for neuropathic pain than either drug alone?

Setting: Outpatient (specialty)

Study Design: Crossover trial (randomized)

Allocation: Concealed

Synopsis: Gabapentin and morphine are widely used for neuropathic pain, but it is unclear whether the combination is better than either drug alone. The authors of this small study used a crossover design. Each patient took each drug or a combination of drugs and served as his or her own control. This study design makes it possible to identify statistically significant results with a relatively small sample size. The 57 patients in the study had diabetic neuropathy or postherpetic neuralgia that was at least moderate in severity and had been present for at least three months. Those with postherpetic neuralgia were somewhat older than those with diabetic neuropathy (mean age: 68 versus 60 years). They stopped taking any medications for neuralgia and kept a pain diary for seven days to establish their baseline level of symptoms.

Patients were then assigned randomly to one of four treatment sequences. Each sequence included the following maximal target dosages for the four treatment regimens: (1) sustained-release morphine in a dosage of 60 mg twice daily, (2) gabapentin in a dosage of 3,200 mg daily in three divided doses, (3) sustained-release morphine in a dosage of 30 mg twice daily plus gabapentin in a dosage of 800 mg three times daily, and (4) active placebo with a low dose of lorazepam (Ativan; not believed to be effective for neuropathic pain, but patients were more likely to believe they were taking an active drug because of its side effects). Each treatment period lasted five weeks, with the dosage slowly escalated during the first three weeks, outcomes measured during the fourth week, and the drugs tapered and stopped during the fifth week. Older and smaller patients had somewhat lower target dosages than the dosages listed above (60 mg for morphine alone and 2,400 mg for gabapentin alone). Most patients did not reach the maximal dosage; the mean final dosages for morphine and gabapentin when used in combination were 35 mg and 1,700 mg per day, respectively.

Only 41 of 57 patients completed the study; most of the others dropped out during the first treatment period. The primary outcome was the mean pain intensity on a scale from zero to 10 during the fourth week when patients were receiving the maximal dosage of each drug. Average pain intensity was 5.70 at baseline and was decreased to 4.50 with placebo, 4.15 with gabapentin, 3.70 with morphine, and 3.10 with the combination of gabapentin and morphine. The differences between the individual active drugs and the combination were statistically significant but of marginal clinical significance. In general, on a 10-point scale, a difference of less than 1 to 1.5 points is not clinically important. Patients receiving morphine alone or in combination with gabapentin had significant side effects; 21 percent receiving the combination had constipation, sedation, and dry mouth.

Bottom Line: The combination of gabapentin and morphine provides a small but clinically unimportant benefit over either drug alone. Tricyclic antidepressants have been shown in other studies to be as effective as gabapentin and are much less expensive, but were not studied in this trial. (Level of Evidence: 1b)

MARK EBELL, M.D., M.S.

Study Reference: Gilron I, et al. Morphine, gabapentin, or their combination for neuropathic pain. N Engl J Med March 31, 2005;352:1324-34. Used with permission from Ebell M. Gabapentin + morphine marginally better than either alone for neuralgia. Accessed online June 1, 2005, at: http://www.InfoPOEMs.com.

COPYRIGHT 2005 American Academy of Family Physicians
COPYRIGHT 2005 Gale Group

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