I'd say you first have to decide just what a p
otion of heroism does or is doing. Is it actually increasing the life energy of the person imbibing it, or simply boosting their abilities to act at that level. The 1E DMG (pg. 126) (being more precise than the 2E DMG) refers to it as:
"...gives the imbiber a temporary increase in life energy levels"'
and then goes on to say:
"When the potion is quaffed, the individual fights as if he or she were at the experience level bestowed by the magic of the elixir."
So it depends on how you interpret what the potion does.
If the potion is merely enhancing the PC's ability to fight and defend himself, then I'd say the potion doesn't count if level drains come into play - the fighter who is originally 5th level and then imbibes a
potion of heroism (thus making him 7th level) and is subsequently touched by a vampire loses 2 levels from his original 5 and is now 3rd level, but would fight as a 6th level fighter due to the effects of the potion until it wears off. After that, he's now 3rd level. The level drain is draining his actual life force, and the potion is not actual life force - it just enhances his ability to act as if he were higher level.
If you consider that the potion actually increases his life force (perhaps if forms some sort of physical link to the Positive Material Plane), then a level drain in this case would be taken from the 2 levels granted by the potion. So in this case, the fighter who is originally 5th level and then imbibes a
potion of heroism (thus making him 7th level) and is subsequently touched by a vampire loses 2 levels from his enhanced 7th level due to the potion, and he is now his normal 5th level.
Alternately, I can see ruling that the potion is magically enhancing that connection to the Positive Material Plane for the duration of the potion's effects, so any touch by the vampire would reduce his enhanced level by 2 for that round alone, but not in subsequent rounds (if the vampire doesn't touch him in the following round) as the potion continues to "feed" him new life force. In this case, the vampire would be leeching off the energy of the potion, not the PC. A second drain in the same round would then either reduce the potion's effects by an additional round, or could be taken from the fighter's actual level, depending how the DM wanted to do it.
Personally, I don't like the second approach. The first one, on the other hand, is pretty rough. So I'd probably go with my suggestion for a third approach, if it ever came up.
For the cleric, I agree mainly with Billy. It lasts only (at max) a little over a hour (80 minutes) with an average time of 6 hours. Not a game breaker. However, while I would allow the cleric or wizard to cast spells as if he were the level granted by the potion, I would
not in any case allow the cleric or wizard to cast spells he can't cast at his normal, pre-potion level. Those classes have to study for and learn spells or have access to them, so the potion would not magically bestow new spells on them. In other words, a 3rd level wizard who drinks the potion and becomes 6th level would not be able to cast
fireball, although he would be able to cast
burning hands doing 1d3 + 12 damage instead of 1d3 + 6 dam. He'd get 3
magic missiles instead of 2, etc. Same for the cleric.
When calculating the chance of spell failure when casting from a scroll, I'd use the wizard's potion-enhanced level to determine the chance of spell failure. So for example, our 3rd level wizard, if he tried casting
fireball off a scroll, would have a 15% chance of spell failure and (if it fails) a 95% chance of total failure with a 5% chance of a reverse or harmful effect. If, however, he drank the potion, he would be casting as a 6th level caster and could cast
fireball off a scroll with no chance of spell failure.
Well, unless it was a cursed scroll.
