While some of the adverse events caused by the administration of medicines are specifically attributable to the drug molecule, a proportion arises because of the chemical, biological and physical nature of the formulation. The effects may be compounded by certain patient factors, an incomplete understanding of the behaviour of the formulation or the coadministration of other drugs. This review examines adverse drug reactions and other adverse events arising from the nature of the dosage form or formulation used. These adverse effects may be the result of local irritation/toxicity, hypersensitivity or allergic reactions, systemic effects from essentially local therapies, or idiosyncratic reactions in a small number of individuals. In certain cases where the exact nature of the formulation is unknown, adverse events cannot be attributed to any single ingredient. In addition, the total of all ingredients of a formulation, even where details of the formulation are clear, may give rise to abnormal behaviour of the formulation in vivo. Often the desired objective of a particular specialised formulation leads to an unforseen but related adverse effect, and in certain instances these events are completely unpredictable and at variance with the perceived objectives of the formulation.