Energy drain and effects on effective level

[Li Shenron]

What does the following effect of energy drain actually affect?

–1 effective level (whenever the creature’s level is used in a die roll or calculation, reduce it by one for each negative level).

At least, caster level comes into my mind, as well as for example special abilities that have a DC based on level. What else?

Does it have any effects on being vulnerable to spells that only affects creatures with lower HD? For example, if you are level 6 (and have 6 HD) and get 2 negative levels, are you suddenly vulnerable to a Sleep spell? At first sight I would say no because you still have 6 HD...

 

[Telperion]

No, I don't think the -1 effective level has any significant affects on a character, other than imposing a penalty on just about any roll you make, losing spells from your head and generally losing level dependant abilities (class or otherwise).

The real problems start if you mess up those Fortitude saves that come after 24 hours of receiving a negative level. That's when you lose HD's and everything else. So, to answer your question: no, you don't suddenly become a victim of the Sleep spell if you gain some negative levels, but should those negative levels turn into permanent level loss...well that's a problem.