The fact of being incompatible.
Condition in which two substances (organisms, tissues, drugs) cannot exist together (e.g., one drug cancels or alters the therapeutic effect of another drug given at the same time).
A state in which two substances cannot be mixed without one of them being changed or destroyed.
The use of two or more drugs which react together to produce adverse consequences for the patient. The British National Formulary carries an appendix devoted to drug interactions.
The quality of not being suitable for mixture. It can be applied to a state that renders admixture of medicines unsuitable through chemical action or interaction, insolubility, formation of poisonous or explosive compounds, difference in solubility, or antagonistic action.