Hereditary coproporphyria (HCP) is the rarest of the autosomal dominant acute porphyrias with an estimated incidence of 0.02 per 10 million per year. HCP has been considered to be mild in presentation compared with the more common acute intermittent porphyria although there is limited information comparing the subtypes. Penetrance in the acute porphyrias is low with 90% of patients with a mutation never exhibiting symptoms. We present seven members from a family with HCP with a novel mutation in whom penetrance and severity are high. In addition, they appear to have a high rate of veno-thromboembolism. Penetrance is confirmed at 57% but is suspected to be 71%. The first patient experienced life-threatening complications, four of the seven have had recurrent attacks and the development of opioid dependence has complicated management. The case series documents the impact of a new mRNA interference molecule givosiran as well as a plan for embryo selection which is not commonly used in porphyria. The use of ketamine for the treatment of acute attacks is also documented for the first time in the porphyria literature. The use of international registries would aid the characterisation and management of this very rare disease.