Be VERY careful when taking prophylactic steroids: there are expected side effects, and there are also a host of ideosyncratic individual response to such medications. And many people don't get altitude sickness at reasonable heights anyway. Climbers usually acclimate for weeks before attempting climbs above the altitude of virtually any generally reachable city/area.

Altitude doesn't eliminate fluid from the body; low humidity does - which obviously often is associated with high altitudes.

Bill New Orleans (Below Sea Level)