 DESIGN This is a prospective observational multicenter study on the pharmacokinetics and pharmacodynamics of paracetamol, fentanyl, midazolam, phenobarbital, doxapram, levetiracetam, sildenafil, ibuprofen and fluconazole routinely administered to preterm infants according to standard protocols. The treatment regimen is left to the discretion of the treating physician in accordance with current guidelines. During this study a limited number of additional blood samples will be drawn from the participants for pharmacokinetic analysis, never exceeding 3 of the total blood volume during any four--week period or 1 at any single time. 