Effect of Oral Cimetidine in the Protoporphyrias

Inclusion Criteria: * Prior enrollment or co-enrollment in the Longitudinal Study of the Porphyrias (PC Study 7201) with a confirmed diagnosis of EPP or XLP * Male or female age ≥15 years at screening * Characteristic history of non-blistering cutaneous photosensitivity * Willing and capable of giving informed consent and following procedures described in the protocol Exclusion Criteria: * Participants not willing to expose themselves to light to the point of prodromal symptoms at least weekly * History of liver or bone marrow transplant or clinically significant liver dysfunction as determined by the Investigator * Known or suspected allergy or intolerance to cimetidine * Use of any other experimental therapy in the past 3 months at screening * Use of cimetidine within the past 3 months at screening * Individuals with elevations of porphyrins in plasma or erythrocytes due to other diseases (i.e., secondary porphyrinemia) such as liver and bone marrow diseases * Patients with any clinically significant comorbid conditions, which in the opinion of the Investigator, precludes participation * Treatment with any drugs or supplements (Appendix 1) that in the opinion of the Investigator can interfere with subject safety or the objectives of the study * The participant either does not have a smartphone or is not willing to use his/her smartphone for the study * Women who are pregnant, breastfeeding, or actively planning to become pregnant * Individuals with moderate to severe ren…