This review illustrates the contribution of a spectroscopic technique (the photochemically induced dynamic nuclear polarization or photo-CIDNP) to the study of the primary radical photoprocesses occurring with several photosensitizing drugs. Examples are given from five series of chemicals: phenothiazines, furocoumarinsm porphyrins, quinolones and sulfamides. CIDNP can afford mechanistic details about the reaction, allow the characterization of unstable intermediates, study the interactions between the drug and biological substrates such as the nucelobases or the amino-acids. It could also constitute a simple tool for testing in vitro the photosensitizing activity of the drug.