The complexities associated with clinical trials for (ultra) rare diseases include regulatory and logistical hurdles and the challenge of building trusting relationships with health authorities, patients, and clinicians. Significant obstacles include the identification of relevant endpoints, sample size limitations, and the cost of maintaining diverse clinical sites. Recruitment and retention of study participants are complicated by site inexperience and the specialised nature of rare disease management, necessitating comprehensive training of site staff and effective communication strategies. All these hurdles directly or indirectly impact the regulatory medical writer preparing complex rare disease clinical documents that comply with regulatory and industry standards. Medical Writing. 2025;34(1):16–20. https://doi.org/10.56012/rqli4755