Under normal circumstances, the flow through the haem synthetic pathway, and therefore of the production of haem, is controlled by the first enzyme of the pathway, ALA synthase. This enzyme is under negative feedback control by haem itself; when haem levels are low, ALA synthase is induced (up-regulated), and there is greater flow though the pathway.

In porphyria, haem levels are often reduced even under normal conditions because of the defect in one or other of the enzymes along the pathway. This results in induction of ALA synthase and increased porphyrin production in an attempt to make haem—this is one of the reasons why porphyrins accumulate in porphyria.

When haem arginate is administered intravenously, it is taken up by the liver and the resulting increase in haem levels is sensed, ALA synthase is down-regulated and the increased flow through the pathway stops. ALA and PBG levels fall rapidly, and clinical improvement follows.