FDA Neurontin Prescribing Information dated 8/10/2011 Reference ID: 2998182 mentions thirst as an "infrequent" side effect of gabapentin.
FDA Neurontin Medication Guide dated 5/1/2013 Reference ID: 3301312 does not mention thirst as a side effect of gabapentin.
Terry et al. "Pharmacokinetic profile and behavioral effects of gabapentin in the horse" J. Vet. Pharmacol. Therap. 33, 485–494. PMID 20840393
Used 20 mg/kg gabapentin iv over 30 min, or po.
Drinking frequency was increased from 19.6 ± SD 7.6 to 25.8 ± SD9.8 bouts over 11 h (but no overt differences in behavior were noticed).
During the 11 h period of behavioral analysis the only changes that were significantly affected by the i.v. dose of gabapentin were drinking frequency and standing rest time, which were increased and decreased, respectively. Gabapentin and the related drug pregabalin have not been reported to increase thirst in other species and hence it remains unknown why the animals drank more frequently. Environmental condi- tions were not controlled, however large fluctuations in ambient temperature were not present.
Kayser and Christensen, Antinociceptive effect of systemic gabapentin in mononeuropathic rats, depends on stimulus characteristics and level of test integration. Pain. 2000 Oct;88(1):53-60. PMID 11098099
Dose response from 1 to 100 mg/kg gabapentin i.p.
In the thermal test, gabapentin (3, 10 and 30 mg/kg i.p.) dose-dependently increased the response time to the 10 degrees C stimulus. Gabapentin at 100 mg/kg but not at 30 mg/kg produced motor deficits in animals using the rotarod test.