Abstract: Methylphenidate (Ritalin) is a CNS stimulant that inhibits dopamine and nor-epinephrine reuptake into presynaptic neurons. It has potential for abuse and diversion. This case-series study has been performed in Tehran University of Medical Sciences. Thirty six medical students of 4 medical schools participated in this study in spring 2004. A questionnaire has been designed for this study for collecting data. This questionnaire was certificated by psychiatric professors of Tehran University of Medical Sciences. Thirty three male and three female students participated in this study. The mean age was 25.1 and 94.4% of the students retried Ritalin for more than one time. Two third of the students had history of other illicit drugs abuse (42.5% alcohol, 28.6% Cannabis). The most common adverse effects after the last time of abusing were increasing in concentration (97.2%), tachycardia (61.1%), restlessness (41.7%), decreasing the appetite (38.4%), anxity (35.7%), diaphoresis (32.5%) this case series shows, unfortunately the average illicit drug abusers was high in participants. Ritalin abuse in medical students is a potentially serious health threat.