Purpose: Patients with erythropoietic protoporphyria (EPP), a severe painful photodermatosis, experience prodromal sensations when exposed to sunlight, which are the “warning signals” to exit the sun, as prolonged exposure causes an excruciatingly painful phototoxic attack. The unique prodromal cutaneous sensations are reversible and differ from the severe burning pain attack lasting 2–7 days. Previously, afamelanotide treatment was studied using time to pain or time outside as primary outcome measures. Since patients have an ingrained fear of sunlight, these measures did not capture the full treatment effect. We retrospectively characterized and evaluated time to prodrome (TTP) as a safer, patient-reported outcome (PRO) measure in afamelanotide-treated patients. Methods: Structured interviews recorded TTP before and during afamelanotide treatment in retrospective US and Dutch cohort studies. Results: Thirty-one US and 58 Dutch EPP patients participated. Before afamelanotide treatment, 54.8% US and 39.7% Dutch patients reported TTP onset <10 minutes in direct sunlight. In both studies, patients’ TTP’s were significantly longer during afamelanotide treatment (p < 0.0001). All US patients’ TTP increased; no TTP was <10 minutes. Among Dutch patients 81% improved; only 10.3% reported TTPs < 10 minutes. Conclusion: EPP patients reported substantial improvements in TTP during afamelanotide treatment. TTP could provide a safer, PRO-based efficacy endpoint for assessing future EPP treatments.