The subject of the spell suffers as his blood heats up, taking 2d6 points of fire damage.
For every level above first, the spell lasts an additional round, dealing another 2d6 points of damage each round. A successful Fortitude save reduces the duration of the spell to 1 round.
If you cast boiling blood on the same subject before the first casting has expired, the second spell has a different effect. In any round in which the subject takes damage from the spell, he is also sickened. This extra effect lasts for as long as the duration of both spells continue, after which only the normal effect occurs for the rest of the duration of the latter spell.
Creatures immune to extra damage from critical hits are immune to this spell, as are creatures that lack blood or similar internal fluids.