The paper points out that the current theories about the effects of psychiatric drugs in healthy human individuals is primarily a function of the methodological approaches applied in psychopharmacological research. Three approaches are distinguished: the diagnostic, the differential-psychological and the experimental-psychological approach. Within the diagnostic approach psychological-diagnostic instruments (e.g. tests) are considered to be able to measure specific effects of drugs (e.g. concentration). The differential-psychological approach takes into account inter- and intraindividual differences in psychological and psychophysiological variables (e.g. personality traits, actual state variables), which are responsible for the kind of drug effect on certain variables. The experimental-psychological approach typically studies drugs from the viewpoint of psychological theories concerning those psychological variables supposed to be affected by the drug (e.g. anxiety, attention). For each of these approaches the theoretical concept, the data gathering procedure, the kind of data interpretation, and the empirical adequacy are discussed. Meaning and usefulness of such terms as "Eigenwirkung", primary and secondary effect, main and side effect, and paradoxical effect are considered.