Excellent points snow. There is also a mild (or not so mild) possibility of shock (the medical condition) associated with sub-drop. It doesn't always happen, but definitely needs to be watched for. If the submissive is prone to suicidal tendencies, sub-drop can trigger such a tendency as well. Immediate after-care from a good session works to reduce the chance of sub-drop (not always, but often). Many subs don't want water or nibblies after a session, but it is a good idea to provide them (and the hugs and gentle words) anyway and make sure they have some water and chocolate and/or fruit during the after care. The sugars in the fruit and chocolate (and the chocolate itself) help reduce the endorphine let down a little. The whole after-care 'gentling process' also has a mental component that counters the depression that may accompany sub-drop (by making pleasant associations with the initial decrease of endorphine levels.

I mentioned shock - many subs (especially in a hard session) get clammy skin and are a little shaky - this is a mild form of shock induced by both the endorphines and the physical 'trauma' of the session. Sips of water and something light (fruit, chocolate, both) help with that as well, as does holding the sub and wrapping her so she's warm ( besides, it feels so good to hold her naked body in my lap and caresses her and tell her what a good girl she is as I feed her nibbles and sips).
Whippett you are so right as well.. i have hit that low of depression when i wasn't taken care of after a session as i needed to be.. only because some Dom's didn't know about sub drop...

It can be very scary for the person going through it and dangerous.. That is why i wanted to point it out clearly that it is a must know if you are playing hard and hit sub space..

i would hate to think sub space would be ruined for anyone by a bad sub drop..

and you are right after a good hard play session it is always nice to be shown a little TLC anyways.. to know you didn't disappiont.. Reassurance is very important to a sub.. after all she is trying to please..