In their recent article, van Gorp et al1 did not specify whether they excluded subjects who met DSM-IV criteria for nicotine dependence. Although subjects were administered a urine screen to examine for substances of abuse, cigarette smoking status was apparently not assessed. Nicotine dependence and withdrawal were considered important confounding factors in a previous study exploring the relationship between hand tremor, frequency of cocaine use, and duration of cocaine abstinence.2 Nicotine has been observed to reduce the tremor of parkinsonism but increase physiologic tremor,3 either of which could have significantly affected the results obtained by van Gorp et al1 on the timed Pursuit Rotor Task. Nicotine stimulates release of dopamine by presynaptic and postsynaptic actions on dopaminergic neurons,3 so alterations in the dopamine system and performance on tasks of psychomotor function in subjects who were dually cocaine- and nicotine-dependent might result from exposure and/or withdrawal from either substance.
Wiseman EJ. Nicotine Dependence and Withdrawal in Cocaine-Dependent Patients. Arch Gen Psychiatry. 2000;57(5):511. doi: