Pethidine is an opioid that gains its popularity for the effective pain control through acting on the opioid-receptors. However, rapid pain relief sometimes brings about unfavourable side effects that largely limit its clinical utility. Common side effects include nausea, vomiting and hypotension. In patients with impaired renal and liver function, and those who need long-term pain control, pethidine may cause excitatory central nervous system (CNS) effects through its neurotoxic metabolite, norpethidine, resulting in irritability and seizure attack. On the contrary, though not clinically apparent, pethidine potentially causes inhibitory impacts on the CNS and impairs normal cerebellar and oculomotor function in the short term. In this case report, we highlight opioid's inhibitory side effects on the cerebellar structure that causes dysmetria, dysarthria, reduced smooth pursuit gain and decreased saccadic velocity.