Gabapentin was first approved for use in 1993. It has been available as a generic medication in the United States since 2004. In 2018, it was the eleventh most commonly prescribed medication in the United States, with more than 45 million prescriptions. During the 1990s, Parke-Davis, a subsidiary of Pfizer, used a number of illegal techniques to encourage physicians in the United States to prescribe gabapentin for unapproved uses. They have paid out millions of dollars to settle lawsuits regarding these activities.
Gabapentin is recommended as a first-line treatment for chronic neuropathic pain by various medical authorities. This is a general recommendation applicable to all neuropathic pain syndromes except for trigeminal neuralgia.
In regard to the specific diagnoses, the best evidence exists for gabapentin treatment of postherpetic neuralgia and diabetic neuropathy. Gabapentin is approved for the former indication in the US. In addition to these two neuropathies, European Federation of Neurological Societies guideline notes gabapentin effectiveness for central pain. A combination of gabapentin with an opioid or nortriptyline may work better than either drug alone.
Overall, gabapentin shows moderate effectiveness for neuropathic pain. Only a minority of patients obtain meaningful relief. Out of 10 persons treated with gabapentin, three to four benefit substantially as compared to one to two persons treated with placebo.
Evidence finds little or no benefit and significant risk in those with chronic low back pain or sciatica. Gabapentin is not effective in HIV-associated sensory neuropathy and neuropathic pain due to cancer.
There is a paucity of research on the use of gabapentin for the treatment of anxiety disorders. In a controlled trial of breast cancer survivors with anxiety, and in a trial for social phobia, gabapentin significantly reduced anxiety levels. For panic disorder, gabapentin is ineffective. On the other hand, some psychiatric textbooks see "a possible role for gabapentin in anxiety disorders, particularly social phobia and panic disorder" based on "case reports and double-blind studies" or state that controlled studies have not shown gabapentin to be very effective for psychiatric indications; "however, clinically it is effective".
Gabapentin is moderately effective in reducing the symptoms of alcohol withdrawal and associated craving. The evidence in favor of gabapentin is weak in the treatment of alcoholism: it does not contribute to the achievement of abstinence, and the data on the relapse of heavy drinking and percent of days abstinent do not robustly favor gabapentin; it only decreases the percent days of heavy drinking.
Gabapentin is ineffective in cocaine dependence and methamphetamine use, and it does not increase the rate of smoking cessation. Gabapentin does not significantly reduce the symptoms of opiate withdrawal. There is insufficient evidence for its use in cannabis dependence.
Gabapentin is recommended as a first-line treatment of the acquired pendular nystagmus, torsional nystagmus, and infantile nystagmus; however, it does not work in periodic alternating nystagmus.
Gabapentin decreases the frequency of hot flashes in both menopausal women and patients with breast cancer. However, antidepressants have similar efficacy, and the treatment with estrogen more effectively prevents hot flashes.
The gabapentin label contains a warning of an increased risk of suicidal thoughts and behaviors. According to an insurance claims database study, gabapentin use is associated with about 40% increased risk of suicide, suicide attempt and violent death as compared with a reference anticonvulsant drug topiramate. The risk is increased for both bipolar disorder and epilepsy patients. Another study has shown an approximately doubled rate of suicide attempts and self-harm in patients with bipolar disorder who are taking gabapentin versus those taking lithium.
Withdrawal symptoms typically occur 1-2 days after abruptly stopping gabapentin. Agitation, confusion and disorientation are the most frequently reported, followed by gastrointestinal complaints and sweating, and more rare tremor, tachycardia, hypertension, and insomnia. In some cases, users experience withdrawal seizures. All these symptoms subside when gabapentin is re-instated.
On its own, gabapentin appears to not have a substantial addictive power. In human and animal experiments, it shows limited to no rewarding effects. The vast majority of people abusing gabapentin are current or former abusers of opioids or sedatives. In these persons, gabapentin can boost the opioid "high" as well as decrease commonly experienced opioid-withdrawal symptoms such as anxiety.
Through excessive ingestion, accidental or otherwise, persons may experience overdose symptoms including drowsiness, sedation, blurred vision, slurred speech, somnolence, uncontrollable jerking motions, and anxiety. A very high amount taken is associated with breathing suppression, coma, and possibly death, particularly if combined with alcohol or opioids.
Gabapentin is a ligand of the ?2? calcium channel subunit. ?2? is an auxiliary protein connected to the main ?1 subunit (the channel-forming protein) of high voltage activated voltage-dependent calcium channels (L-type, N-type, P/Q type, and R-type). Gabapentin is not a direct channel blocker: it exerts its actions by disrupting the regulatory function of ?2? and its interactions with other proteins. Gabapentin prevents delivery of the calcium channels to the cell membrane, reduces the activation of the channels by the ?2? subunit, decreases signaling leading to neurotransmitters release, and disrupts interactions of ?2? with NMDA receptors, neurexins, and thrombospondins. Out of the four known isoforms of ?2? protein, gabapentin binds with similar high affinity to two: ?2?-1 and ?2?-2. Most of the pharmacological properties of gabapentin are explained by its binding to just one isoform - ?2?-1.
Gabapentin is a potent activator of voltage-gated potassium channels KCNQ3 and KCNQ5, even at low nanomolar concentrations. However, this activation is unlikely to be the dominant mechanism of gabapentin's therapeutic effects.
The oral bioavailability of gabapentin is approximately 80% at 100 mg administered three times daily once every 8 hours, but decreases to 60% at 300 mg, 47% at 400 mg, 34% at 800 mg, 33% at 1,200 mg, and 27% at 1,600 mg, all with the same dosing schedule. Drugs that increase the transit time of gabapentin in the small intestine can increase its oral bioavailability; when gabapentin was co-administered with oral morphine, the oral bioavailability of a 600 mg dose of gabapentin increased by 50%.
Gabapentin at a low dose of 100 mg has a Tmax (time to peak levels) of approximately 1.7 hours, while the Tmax increases to 3 to 4 hours at higher doses. Food does not significantly affect the Tmax of gabapentin and increases the Cmax and area-under-curve levels of gabapentin by approximately 10%.
Gabapentin is eliminatedrenally in the urine. It has a relatively short elimination half-life, with the reported average value of 5 to 7 hours. This value changes with increasing doses, from 5.4 hours for a 200 mg single dose, to 8.3 hours for a 1,400 mg dose. Because of its short elimination half-life, gabapentin must be administered 3 to 4 times per day to maintain therapeutic levels. Gabapentin XR (brand name Gralise) is taken once a day.
Chemical structures of GABA and gabapentin, with commonalities highlighted
Gabapentin was designed by researchers at Parke-Davis to be an analogue of the neurotransmitter GABA that could more easily cross the blood-brain barrier and was first described in 1975 by Satzinger and Hartenstein. Under the brand name Neurontin, it was first approved in May 1993, for the treatment of epilepsy in the United Kingdom. Approval by the U.S. Food and Drug Administration followed in December 1993, for use as an adjuvant (effective when added to other antiseizure drugs) medication to control partial seizures in adults; that indication was extended to children in 2000. Subsequently, gabapentin was approved in the United States for the treatment of postherpetic neuralgia in 2002. A generic version of gabapentin first became available in the United States in 2004. An extended-release formulation of gabapentin for once-daily administration, under the brand name Gralise, was approved in the United States for the treatment postherpetic neuralgia in January 2011.
Although some small, non-controlled studies in the 1990s--mostly sponsored by gabapentin's manufacturer--suggested that treatment for bipolar disorder with gabapentin may be promising, the preponderance of evidence suggests that it is not effective. Subsequent to the corporate acquisition of the original patent holder, the pharmaceutical company Pfizer admitted that there had been violations of FDA guidelines regarding the promotion of unproven off-label uses for gabapentin in the Franklin v. Parke-Davis case (see below).
Reuters reported on 25 March 2010, that "Pfizer Inc violated federal racketeering law by improperly promoting the epilepsy drug Neurontin ... Under federal RICO law the penalty is automatically tripled, so the finding will cost Pfizer $141 million." The case stems from a claim from Kaiser Foundation Health Plan Inc. that "it was misled into believing Neurontin was effective for off-label treatment of migraines, bipolar disorder and other conditions. Pfizer argued that Kaiser physicians still recommend the drug for those uses", and that "the insurer's website also still lists Neurontin as a drug for neuropathic pain."
The Wall Street Journal noted that Pfizer spokesman Christopher Loder said, "We are disappointed with the verdict and will pursue post-trial motions and an appeal." He later added that "the verdict and the judge's rulings are not consistent with the facts and the law."
Franklin v. Parke-Davis case
While off-label prescriptions are common for a number of drugs, marketing of off-label uses of a drug is not. In 2004, Warner-Lambert (which subsequently was acquired by Pfizer) agreed to plead guilty for activities of its Parke-Davis subsidiary, and to pay $430 million in fines to settle civil and criminal charges regarding the marketing of Neurontin for off-label purposes. The 2004 settlement was one of the largest in U.S. history, and the first off-label promotion case brought successfully under the False Claims Act.
Gabapentin was originally marketed under the brand name Neurontin. Since it became generic, it has been marketed worldwide using over 300 different brand names. An extended-release formulation of gabapentin for once-daily administration was introduced in 2011 for postherpetic neuralgia under the brand name Gralise.
Gabapentin when taken in excess can induce euphoria, a sense of calm, a marijuana-like high, improved sociability, and reduced alcohol or cocaine cravings. Also known on the streets as "Gabbies", gabapentin is increasingly being abused and misused for these euphoric effects. Withdrawal symptoms, often resembling those of benzodiazepine withdrawal, play a role in the physical dependence some users experience. About 1 percent of the responders to an internet poll and 22 percent of those attending addiction facilities had a history of abuse of gabapentin. Its misuse predominantly coincides with the usage of other illicit CNS depressant drugs, namely opioids, benzodiazepines, and alcohol.
After Kentucky's implementation of stricter legislation regarding opioid prescriptions in 2012, there was an increase in gabapentin-only and multi-drug use in 2012-2015. The majority of these cases were from overdose in suspected suicide attempts. These rates were also accompanied by increases in abuse and recreational use. Gabapentin misuse, toxicity, and use in suicide attempts among adults in the US increased from 2013 to 2017.
In cats, gabapentin can be used as an analgesic in multi-modal pain management, anxiety medication to reduce stress in cats for travel or vet visits, and anticonvulsant. Veterinarians may prescribe gabapentin as an anticonvulsant and pain reliever in dogs. It is also used to treat chronic pain-associated nerve inflammation in horses and dogs. Side effects include tiredness and loss of coordination.
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