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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.

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Gabapentin 2-mg/mL Injection, Preservative Free
From International Journal of Pharmaceutical Compounding, 1/1/05

METHOD OF PREPARATION

Note: This preparation should be prepared in a laminar airflow hood in a cleanroom or vin isolation barrier technology by a validated aseptic compounding pharmacist 'using strict aseptic technique. This is a high-risk preparation.

1. Calculate the required quantity of each ingredient for the total amount to be prepared.

2. Accurately weigh and/or measure each ingredient.

3. Dissolve the gabapentin and sodium chloride in sufficient sterile water for injection to volume and mix well.

4. Filter through a sterile 0.20 -µm filter into individual-dose sterile vials or single-use syringes.

5. Package and label.

PACKAGING

Package in tight, light-resistant containers.1

LABELING

Keep out of reach of children. Use only as directed. Store in a refrigerator.

STABILITY

Tf not sterility tested: A bcyond-usc date of up to 24 hours at room temperature, up to 3 days at refrigerated temperature (2 to 8°C), or up to 45 days if frozen can be used for this preparation.1

If sterility tested: A beyond-use date of up to 14 days stored in a refrigerator can be used for this preparation.1

USE

Gabapentin injection is used in the treatment of neuropathic pain.

QUALITY CONTROL

Quality-control assessment can include weight/volume, physical observation, pH, specific gravity, osmolality, assay, color, clarity, paniculate matter, sterility and pyrogenicity.2,3

DISCUSSION

Gabapentin is an anticonvulsant structurally related to the central nervous system neurotransmitter γ-aminobutyric acid. It is used as an antiepileptic that is effective in the treatment of partial seizures with or without secondary generalization and is used as adjunctivc therapy in patients unresponsive to or intolerant of standard antiepileptic drugs. It is vised in the treatment of neuropathic pain in adults, with doses titrated to a maximum of 1.8 g daily in three divided doses. In renal impairment, reduced doses are recommended. It has also been used in the treatment of headache, postherpetic neuralgia, multiple sclerosis and Parkinsonism.4

Gabapentin (C^sub 9^H^sub 17^NO^sub 2^, MW 171.24) is not an official substance in the United States Pharmacopeia. It occurs as white crystals that melt at about 164 to 167°C. Tt has two pK^sub a^ values, one at 3.68 and one at 10.7. It is soluble greater than 100 mg/mL in water at pH 7.4.5

Sodium chloride (NaCl, MW 58.44) is available as a white crystalline powder or as colorless crystals. It has a saline taste and is used in a variety of parenteral and nonparenteral pharmaceutical formulations. In parenteral, ophthalmic and nasal preparations, it is used to prepare isotonic solutions. It is also used as a capsule diluent and lubricant and as a means of controlling drug release from some microcapsules, controlling micelle size and adjusting the viscosity of some polymer dispersions by altering the ionic character of the formulation. The pH of a saturated solution is in the range of 6.7 to 7.3, and it is soluble in water (l g in 2.8 m L), glycerin (1 g in 10 mL) and 95% ethanol (l g in 250 mL). A 0.9% w/v aqueous solution is iso-osmotic with serum, and its solutions are stable. Its solutions are corrosive to iron and will react to form precipitates with silver, lead and mercury salts. Chlorine can be liberated in the presence of strong oxidizing agents from its acidified solutions. It can also decrease the solubility of methylparaben in aqueous solution.6

Sterile water for injection is water for injection that has been sterilized and suitably packaged; it contains no added substances.1

REFERENCES

1. United States Pharmacopeial Convention, Inc. United States Pharmacopeia 27-Nationa/Formulary22. Rockville, MD: US Pharmacopeial Convention, Inc.; 2004: 785-786,1950,2345-2349.

2. Alien LV Jr. Standard operating procedure for particulate testing for sterile products. /JPC1998; 2(1): 78.

3. Alien LV Jr. Standard operating procedure: Quality assessment for injectable solutions./JPC1999; 3(5): 406-407.

4. Sweetman SC, ed. MAHTINDALE: The Complete Drug Reference. 33rd ed. London: The Pharmaceutical Press; 2002: 350-351.

5. White HS. Antiepileptic drugs. In: Gennaro AR. Remington: The Science and Practice of Pharmacy. 20th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2000:1425.

6. Owen SC. Sodium chloride. In: Rowe RC, Sheskey PJ, Weller PJ, eds. Handbook of Pharmaceutical Excipients. 4th ed. Washington, DC: American Pharmaceutical Association; 2003: 556-559.

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