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.