The acute porphyrias are a group of rare metabolic disorders of the heme biosynthetic pathway. Carriers of the acute porphyria gene are prone to potentially fatal acute attacks, which can be precipitated by drug exposure. It is therefore important to know whether a drug is safe for carriers of the acute porphyria gene. In this study, radiopharmaceuticals were assessed on their porphyrogenicity (ie, the potential of a drug to induce an attack).The assessment was conducted by classifying the drugs according to the Thunell model.From 41 radiopharmaceuticals assessed, I-131 norcholesterol, Tc-99m mebrofenin, Tc-99m phytate, Tc-99m sestamibi, and Tl-201 chloride were classified as possibly porphyrogenic.I-131 norcholesterol, Tc-99m mebrofenin, Tc-99m phytate, Tc-99m sestamibi, and Tl-201 chloride should not be prescribed for patients experiencing acute porphyria unless an urgent indication is present and no safer alternative is available. In such cases, potential users should seek advice from a porphyria expert. Preventive measures may also be required.