This is a double-blind, randomized, placebo-controlled, parallel group trial investigating the efficacy and safety of Panhematin™ for preventing acute attacks in at least 20 patients with well-documented acute porphyria (acute intermittent porphyria, hereditary coproporphyria or variegate porphyria). These patients will (1.) have had frequent attacks in the past, with symptoms such as abdominal, back and/or limb pain and diagnosed after exclusion of other causes, and (2.) be on hemin prophylaxis for prevention of frequent attacks. It is expected that patients will have had 6 or more attacks in one year before starting hemin prophylaxis. This would be considered justification for a preventive regimen of hemin on clinical grounds. A single double blind dose of Panhematin™ or placebo will be given. An interim analysis will be carried out after completion of 10 patients to assess progress and possibly adjust the sample size. The trial consists of the following: