milk was formerly indicated in some poisonings where it could function as a diluting agent for irritants in the stomach (acids, alkalis, solvents) but nowadays its use is superceded by activated charcoal in almost all medical contexts
it is sort of useful to moderate stimulant experiences as it is a nice rapidly availiable solution of sugars, fats, and protiens (I think that low blood sugar fwhen using stimulants can cause a lot of negative effects from feeling unusually tired to becoming anxious or irritated) but don't make yourself sick drinking too much, and don't expect it to halt a trip at all
if someone is overdosing on opioids, never give anything by mouth as you just add a risk of choking/aspirration. naloxone IM or nasal apray, with airway support via bag valve mask or mouth to mouth until the paramedics can take over.