Hollak's research career has focused mainly on rare metabolic disorders in adults. She is a pioneer in this field, having conducted numerous studies on Gaucher disease and Fabry disease in particular. She also initiated and edited the first international textbook on metabolic disorders in adults. In 2004, she set up an outpatient clinic and, together with other university medical centres, developed a network to draw up guidelines for treating metabolic disorders in adults.
Hollak's long experience investigating drugs for rare diseases has enabled her to see both the advantages and disadvantages of the current system of drug development, accessibility and affordability. She conducts research into that system and makes suggestions for improvements. Hollak gained national fame for ensuring, together with pharmacist Marleen Kemper, that a medicine for a rare disease remained available through a pharmacy preparation. In 2019, Hollak’s involvement led to the establishment of the Medicine for Society (Medicijn voor de Maatschappij) platform, which aims to improve the availability of these ‘orphan drugs’. Hollak links science to practice, from academic product development to regulatory access processes.