Posaconazole is used for the prophylaxis and treatment of invasive fungal infections (IFIs). The development of IFIs in patients being treated for haematological malignancies is associated with increased morbidity and mortality. As diagnosis of IFIs may be difficult, targeted prophylactic therapy is important in high-risk patients. Posaconazole is a more potent antifungal agent and studies suggest it is more effective than earlier triazole antifungal drugs, such as itraconazole. However, more serious side effects are associated with its use. The bioavailability of posaconazole is known to be influenced by food, administration with a fatty meal increases plasma posaconazole concentrations up to 4-fold. Co-admistration of some other drugs, particularly proton pump inhibitors and histamine receptor antagonists, may reduce posaconazole plasma concentration. Thus monitoring plasma concentration may help in dose optimization and in monitoring adherence to medication. A pre-dose target plasma posaconazole concentration of 0.5 mg/L or above has been suggested for effective prophylaxis in patients being treated for haematological malignancies.