In all cases you will receive advice and help from the responsible youth welfare office and the addiction counseling centers.
Youth welfare offices Niedersachsen: https://www.kinderinfo.de/jugendaemter/niedersachsen/
Important information can be found here: https://www.suchtfragen.de/ and https://www.hls-online.org/
Addiction counseling centers in Niedersachsen can be found at: https://nls-online.de/home16/index.php/adressen/56-sucht-und-drogenberatungsstellen