general principles

Has the diagnosis been established?(more about laboratory diagnosis). Individuals may have a family history of porphyria but have not themselves been investigated or a past diagnosis may be uncertain.

If treatment must be started before the diagnosis can be confirmed or refuted, regard all individuals with a family history or past diagnosis however uncertain these may be as affected.

When prescribing in acute porphyria, the benefit from using the drug of choice should always be assessed against the risk of provoking an acute attack and the likely consequences of not using it. A drug should never be withheld if it is judged to be essential for optimum treatment. As an example, Tamoxifen and other anti-neoplastic agents that are potentially unsafe come into this category.
For potentially unsafe drugs, that appeared necessary for the treatment of intercurrent severe diseases in porphyric individuals, monitoring urinary PBG excretion should be done in collaboration with a specialist porphyria centre to detect any drug-provoked increase.

Individuals may be encountered who have used an unsafe drug without adverse effects, even for many years, before being found – usually through family screening - to have latent porphyria. After discussion of the risk, such individuals often wish to continue on the drug and this should be allowed. However, if an additional drug needs to be prescribed, the possible risk of interaction provoking an acute attack needs to be carefully considered and a change to a safer combination may be required.

Individuals who have inherited one of the acute porphyrias vary in their susceptibility to drug-induced acute attacks. It is known that many such individuals tolerate drugs known to be unsafe without adverse reaction. At present there is no way to identify in advance inter-individual variation in the response to drugs. Probable lower risk groups are those aged over 40 years who have never had symptoms, particularly men, those over 40 years who have normal PBG excretion. Acute attacks are very rare in prepubertal children but the usual practice is to prescribe as for young adults.

All individuals starting therapy should be advised to report any adverse reaction (eg abdominal pain) immediately. For all except safe/very low risk drugs, facilities should be available for prompt diagnosis and treatment of an acute attack, including 24 hour access to a supply of human hemin (more about treatment of the acute attack).