Not all people who inherit a gene mutation for one of the acute porphyrias will develop an acute attack. It is estimated that at least three-quarters of people who inherit an acute porphyria gene will never experience an acute attack of porphyria. In those who do become ill, it appears that additional factors are usually required for an attack to occur. Our knowledge of these factors is incomplete but among the most important are a number of drugs, some of which are widely, and alcohol. However acute attacks do occasionally occur in the absence of any identifiable provoking factor. Experience over many years has shown that if people who have inherited one of the acute porphyrias are careful to avoid certain drugs, alcohol and other known factors that may provoke an acute attack, their chance of becoming ill is much reduced.