The attention deficit disorder with hyperactivity (ADDH) is a widely recognized disorder of unknown etiology. Methylphenidate administration is one of the most commonly used treatments to improve symptoms associated with ADDH. Although it is generally a well tolerated drug, several secondary effects may occur. In particular, this paper will focus on the effects on anxiety, in humans and experimental animal models. It has been shown that acute administration of methylphenidate in adults reduces anxiety, in both animal models and humans. On the other hand, chronic treatment during early ages (postnatal and young subjects) results in higher anxiety in adults. In some cases this effect appears together with higher susceptibility of drug consumption. Thus, we find that, in the literature, methylphenidate is capable of inducing different and opposite effects. Thus, further experiments would be required to elucidate the mechanisms by which methylphenidate exert its actions.