 This study will provide information on the pharmacokinetics, safety and effectiveness of off--label drugs used in critically ill premature infants: doxapram, fentanyl, midazolam, paracetamol, phenobarbital, sildenafil, levetiracetam, ibuprofen and fluconazole only studied for the PK. PKPD analysis will result in adapted dosage guidelines, thus contributing towards an improvement in the quality of care and cost efficiency. Furthermore the development of Dried Blood Spot DBS analysis is investigated for these drugs as a minimally invasive method for conventional patient care and to perform pharmacological studies in children. The adapted dosage guidelines will be implemented directly into clinical practice in collaboration with the NKFK. Therefore the study is designed as an observational multicenter study to be able to collect sufficient data for the drugs of interest. 