Zamorak brews temporarily raise Attack and Strength by 8% + 1. It also temporarily lowers Defence by 8% + 1 and remaining Life Points by 12% (all numbers rounded down).
This potion is frequently used in conjunction with restore potions to raise lowered Defence. Contrary to popular belief, this cannot be used in conjunction with a Saradomin brew to raise all four skills as they will cancel each other out.
People would sometimes use this potion to lower their life points to either kill themselves and fully restore their stats, to respawn somewhere else, or to hit higher using Dharok's set effect. It's no longer possible to kill yourself with a Zamorak brew.
Previously, buying torstol potion (unf)s is highly recommended to avoid losing massive amounts of money. Even while doing this, a player will still lose 29.49 coins per xp, so this is not a recommended training method which would mean spending 336,356,555 coins from 78 to 99, an unnecessarily large amount.
The addition of overload has more or less made the Zamorak brew obsolete, as the former has a far more powerful effect and requires a clean torstol herb (unfinished potion doesn't work).
Torstol prices has since fallen to be in line with torstol potions, although Zamorak brews remain more expensive than potions at its level to make.