Acute Intermittent Porphyria is a rare inherited disorder caused by a partial lack of an enzyme needed to make heme, a substance that carries oxygen to all parts of your body. This enzyme deficiency results in the build-up of certain chemicals in your body, causing symptoms to develop. "Acute intermittent" means that symptoms, or "attacks," may occur for a set period of time, then go away, only to return later. Left untreated, AIP attacks can cause damage to your body such as to your brain and nervous system.