Drug-induced interstitial lung diseases present with a variety of tissue reactions, most of them already discussed in previous chapters. A common presentation is diffuse alveolar damage as an acute reaction, often followed by organizing DAD, where hyaline membranes can still be recognized, and finally ending as organizing pneumonia and lung fibrosis. Other drugs induce NSIP-like tissue reactions. The major problem in interpreting drug reactions in the lung is our limited understanding of drug metabolism. Some drugs will induce toxic injury of endothelial cells, thus the blood barrier is leaking, and proteins can enter the interstitium and finally the alveolar lumen. Here these proteins will form complexes and by the action of respiration, hyaline membranes will form. In addition, the exudate from the capillaries will cause a transient edema and this is followed by hypoxia affecting the pneumocytes. Later on, organizing pneumonia can result. Other drugs act on the immune system, forming immune complexes either because immunogenic by itself, or by complexing with endogenous proteins like a hapten.