Whenever there are several strengths or when several single units can be taken simultaneously to achieve the desired dose, dose proportionality for different strengths / doses of the modified release formulations should be adequately addressed. Dose proportionality should be evaluated by means of a single dose and, in case of drug accumulation, a multiple dose study, where the PK parameters of interest of all the strengths/doses are compared after dose adjustment. The criteria described in the Guideline on the Investigation of Bioequivalence (CPMP/EWP/QWP/1401/98) for dose proportionality based on AUC only and 25% acceptance range are not applicable in this case since these criteria only apply for strength selection for bioequivalence studies.