4. Concomitant medications: Systemically absorbed medication will be exclusionary, while non-systemically absorbed medications will generally be permitted. Children who have taken any experimental medication in the past month will be excluded. Any prior psychotropic medication must have a suitable washout period of at least 4 half-lives before assessment can begin (generally a minimum of 2 weeks; up to 4 weeks for fluoxetine). Children taking methylphenidate at the time of inquiry may be withdrawn from medication only if treatment response is less than optimal and a re-evaluation is deemed clinically necessary. Children taking other psychotropic medication may be discontinued, provided their prescribing physician is in agreement with this plan, and provided there is clinical indication for doing so (i.e., either inadequate response, adverse effects or problematic time-action properties).