Convulsions (seizures): Rare [18,53-54]. Seizures are more likely to develop in susceptible individuals such as epileptics or if the seizure threshold is reduced by other drugs or non-drug factors (see factors reducing the seizure threshold).
Data on this topic is conflicting. In most trials on rodents, DXM was shown to have anticonvulsant effects [55-58], although proconvulsant (seizure-potentiating) effects were observed in two trials (on rats) [59-60]. In another trial on rats, DXM was shown to be anticonvulsant at low doses, but induced seizures at high doses [61]. In humans, daily doses of 160-200 mg DXM were shown to have anticonvulsant effects which were more pronounced in intermediate and poor metabolizers at CYP2D6 [54]. However, a moderate amount of DXM (two doses of 300 mg, taken several hours apart) in a CYP2D6 poor metabolizer was reported to cause seizures [18]. In a human trial, DXM in daily doses of 120 mg increased the frequency of complex partial seizures by 25% [53]. In another trial on brain-damaged children suffering from seizures, doses of 20-42 mg/kg DXM (with assisted respiration) had anticonvulsant effects [62].