If I have a prescription for Concerta 18 mg, can I ask for a replacement in the Medikinet 20 mg CR pharmacy?

This will not be possible because the listed products are not substitutes in the pharmaceutical sense. Both contain methylphenidate hydrochloride, but differ not only in the nominal dose, but also in the pharmacokinetic profile, i.e. parameters such as the maximum concentration, the time after which it is reached or the half-term. Such a change could only be made by a doctor simply writing you a new prescription: https://www.gdziepolek.pl/artykuly/czym-sa-zamienniki-lekow The last information is that the new supply of The drug Concerta 18 and 36 mg to pharmaceutical wholesalers is to be carried out. Currently, several outlets integrated with GdziePoLek in the Kujawsko-Pomorskie Voivodeship still have individual packages in stock: https://www.gdziepolek.pl/produkty/19569/concerta-tabletki/apteki/w-kujawsko-pomorskim I encourage you to sign up for notifications about the preparation. The option is available from the link above. I attach additional material, perhaps it will be interesting: https://www.gdziepolek.pl/kategorie/adhd https://www.gdziepolek.pl/artykuly/adhd-choroba-czy-wymysl