Long term use of pethidine is avoided because a metabolite of pcthidine is associated with:

(Ref: KDT 6/e p459) Pethidine is mainly metabolized by hydrolysis to meperidinic acid but a minor pathway of metabolism involving methylation to nor-pethidine is also present. In overdose of pethidine, this minor pathway assumes impoance and there is accumulation of nor-pethidine. This compound possesses excitatory propeies and can lead to tremors, mydriasis, delirium, myoclonus and convulsions.