- For the cocaine sub study: Cocaine users will be excluded for current use of prescription or illicit psychoactive drugs other than cocaine and marijuana, current or past substance dependence criteria other than cocaine, marijuana, or alcohol, smoking >1 pack of cigarettes per day, current breath alcohol concentration >0.002, a lifetime history of head injury, a history of seizures or migraine headaches (as transcranial magnetic stimulation may exacerbate these conditions), and violation of other magnetic resonance imaging safety measures.