Today's Veterinary Practice

NOV-DEC 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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FEATURES NOVEMBER/DECEMBER 2018 29 is not controlled by anti-inflammatory drugs, opioids, or other drugs/drug classes. Which Patients Should Receive Gabapentin? Gabapentin should be administered to patients with a known neuropathic lesion (e.g., extruded disc, nerve injury), suspected neuropathic lesion (e.g., painful back, neck), or chronic pain that is not controlled by anti-inflammatory drugs or for which these drugs are contraindicated. What Is the Dosage for Gabapentin? Gabapentin is often overlooked because it has a reputation for a prolonged onset of action and/ or ineffectiveness. Both of these characteristics can be true, but both may also result from inadequate dosing or dosing regimens, and it is my experience that inadequate dosing is very common. Most of us learned to prescribe gabapentin at 5 mg/kg PO q12h; however, although this regimen may be effective for some patients, neither the dose nor administration frequency is adequate for many patients. Dose and administration frequency can vary among individuals, meaning that we may have to work a little bit to find each patient's optimal dose ( BOX 1 ). But finding the optimal dose is not all that hard to do; we just need to understand the pharmacokinetics of the drug—and enlist the pet client's help. Why is the dose so variable? To answer that, let's first look at data from human medicine, for which the pharmacokinetics are better defined. After oral administration, gabapentin is absorbed from the gastrointestinal tract via an L-amino acid transport system, which is saturable, meaning that as the dose of gabapentin increases, the serum concentration does not increase linearly. In humans, as the dose increases from 900 to 3600 mg/day, the bioavailability of gabapentin drops from 60% to 33%. 11 We know that this same pharmacokinetic phenomenon also occurs in dogs, 12 and we presume that it occurs in cats, although this phenomenon has yet to be studied in cats. What we do know about cats is that the bioavailability of gabapentin is highly variable, at least in nonfasted cats. 13 This finding means that dosages that provide serum concentrations of gabapentin adequate for analgesia may be higher and more variable among patients than we once thought. The dose range has long been reported as 3 to 20 mg/kg, but use of doses as high as 50 mg/kg has been anecdotally reported. Although your initial dose may not be 50 mg/kg, you may eventually reach that dose as you challenge the effective dose for any individual patient. In addition to escalating doses, more frequent administration may be necessary. The pharmacokinetics for dogs 12 and cats 13 indicate that gabapentin administration every 6 to 8 hours, rather than every 12 hours as commonly used, may be needed to provide serum concentrations of gabapentin that are adequate for analgesia. Thus, for each patient, both the dose and the administration frequency may need to be explored, for which we need to engage the client/caregiver. Mild sedation, which can be easily recognized by clients, can be used to guide the need for dosage changes. Working with the client, choose a reasonable endpoint for determining effective pain relief (e.g., taking longer walks, climbing stairs) and discuss the signs of sedation. Then choose a starting dose and, by placing frequent phone calls to the client to check on progress, guide the client on titrating the dose of gabapentin until either endpoint—sedation or decreased pain—is reached. Clients should also be advised that ataxia might occur at higher dosages, especially in large breed dogs and/ or dogs with decreased muscle mass. If ataxia occurs, the same guidelines as described for the occurrence of sedation (e.g., decreasing the dose) should be followed. What Efficacy Data Are Available for Gabapentin? No controlled research studies on the use of gabapentin for the treatment of chronic pain in dogs and cats have been performed. Unfortunately, lack of research is a common problem for most methods of chronic pain treatment in veterinary medicine. Several case reports note analgesia when gabapentin was used for treatment of chronic pain. 14,15 And in a clinical study Working with the client, choose a reasonable endpoint for determining effective pain relief (e.g., taking longer walks, climbing stairs) and discuss the signs of sedation.

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