Memantine in the form of the free base or of a physiologically compatible salt for use in a therapeutic method for treating or preventing Alzheimer's disease, characterized in that it is present in a transdermal therapeutic system, which has - a backing layer impermeable to active ingredient; - at least one active-ingredient-containing layer which comprises a storage material into which memantine in the form of the free base or of a physiologically compatible salt is introduced as active ingredient; - optionally a release liner, - a t max of at least 24 h, where t max is defined as the time which has elapsed before the maximum active ingredient concentration in the blood plasma is reached during the one-off or first administration of the Transdermal Therapeutic System, measured starting from the application of the Transdermal Therapeutic System to the skin, wherein the storage material - comprises a preparation which is in liquid or gel form and in which the active ingredient is partly or completely dissolved; and/or - comprises a matrix which is based on a polymeric matrial and in which the active ingredient is dissolved or distributed as a microdispersion; and/or - is a lipogel which contains from 10 to 50% by weight of active ingredient, and wherein consecutive or staggered administration of two or more transdermal therapeutic systems, which in each case contain the same or a different daily releasable dose of active ingredient, for administration intervals each lasting at least 12 h independently of one another achieves, in the blood plasma of the individual requiring treatment, the following active ingredient concentrations from the beginning in each case of the first administration: after 24 h, at least 14 ng/ml, after 48 h, at least 27 ng/ml, after 60 h, at least 32 ng/ml, after 72 h, at least 37 ng/ml, after 96 h, at least 44 ng/ml, after 144 h, at least 54 ng/ml, after 192 h, at least 60 ng/ml, after 240 h, at least 63 ng/ml, and after 300 h, at least 65 ng/ml, wherein the period between the removal of a Transdermal Therapeutic System at the end of an administration interval and the application of the following Transdermal Therapeutic System is no more than 10 minutes. - a backing layer impermeable to active ingredient; - at least one active-ingredient-containing layer which comprises a storage material into which memantine in the form of the free base or of a physiologically compatible salt is introduced as active ingredient; - optionally a release liner, - a t max of at least 24 h, where t max is defined as the time which has elapsed before the maximum active ingredient concentration in the blood plasma is reached during the one-off or first administration of the Transdermal Therapeutic System, measured starting from the application of the Transdermal Therapeutic System to the skin, wherein the storage material - comprises a preparation which is in liquid or gel form and in which the active ingredient is partly or completely dissolved; and/or - comprises a matrix which is based on a polymeric matrial and in which the active ingredient is dissolved or distributed as a microdispersion; and/or - is a lipogel which contains from 10 to 50% by weight of active ingredient, after 24 h, at least 14 ng/ml, after 48 h, at least 27 ng/ml, after 60 h, at least 32 ng/ml, after 72 h, at least 37 ng/ml, after 96 h, at least 44 ng/ml, after 144 h, at least 54 ng/ml, after 192 h, at least 60 ng/ml, after 240 h, at least 63 ng/ml, and after 300 h, at least 65 ng/ml,