I would assume that that would be the case, yes. Their CON score probably goes down, reducing their maximum hp, so if they tried to heal themselves, it wouldn't be as effective.
Level drain works on monsters in the same way as on characters. It thus doesn't affect statistics that don't change with level (including strength, constitution etc), but does affect all attributes linked to level - HPs, THAC0 and saving throws.
You should particularly note that when anything is level drained their attributes are reduced to the standard values for their new level, totally ignoring any equipment that normally provides special bonuses to that. That can make level draining very dangerous for the PC as saving throws will change greatly as a result of losing a single level. On the other hand the same thing happens to monsters that come equipped with items granting them special bonuses. Draining a level from a dragon for instance could reduce their THAC0 by about 5 as a result of depriving them of the benefit of equipment worn .
It's also worth noting that any HP drained is drained both from maxHP and current HP. So a fighter with 16CON losing one early level (level 9 to 8 for example) will lose 12 current HP, on top of the damage dealt by the attackitself
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You should particularly note that when anything is level drained their attributes are reduced to the standard values for their new level, totally ignoring any equipment that normally provides special bonuses to that. That can make level draining very dangerous for the PC as saving throws will change greatly as a result of losing a single level. On the other hand the same thing happens to monsters that come equipped with items granting them special bonuses. Draining a level from a dragon for instance could reduce their THAC0 by about 5 as a result of depriving them of the benefit of equipment worn