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Gabapentin and pregabalin are fully subsidised on prescription in New Zealand. Originally developed as anti-convulsants, they are more widely used for neuropathic pain. The proposed mechanism for analgesic activity is binding to the a2S subunit of voltage-gated calcium channels in the central nervous system, which reduces the release of excitatory neurotransmitters such as glutamate.1 Numbers needed to treat for 50% reduction in neuropathic pain are reported as 6.3 for gabapentin (5-0-8-3) and 7.7 for pregabalin (6.5-9.4), and are dose-related.2 Switching from gabapentin to pregabalin may be considered for efficacy or tolerability reasons. Although there is no clear evidence that either gabapentin or pregabalin is more effective than the other for neuropathic pain,1 patients may benefit from switching. In an open-label study, analgesia improved after switching from gabapentin to pregabalin.3
There is no established guidance on converting between gabapentin and pregabalin.4 The manufacturers of both pregabalin and gabapentin advise that if they are to be stopped or changed to another medication, the dose should be tapered gradually over at least one week.4 This gradual withdrawal is to minimise the risk of seizures where they are being used for patients with seizure disorders.4 The importance of a slow withdrawal in patients with neuropathic pain remains unknown,4 however discontinuation symptoms have been reported with abrupt cessation of both gabapentin and pregabalin. Discontinuation symptoms reported include insomnia, nausea, anxiety, pain, and sweating.4 Determining dose equivalence between gabapentin and pregabalin is complicated by gabapentin's nonlinear bioavailability, in contrast to pregabalin's linear bioavailability.6 Gabapentin's bioavailability ranges from 80% with 100mg tds6 to 35% with 1,200mg tds.7 Despite this, most...