More and more numerous each year, and only rarely beneficial, these are responsible for unexplained therapeutic failures or complications. They may occur at different levels. In the bottle of infusion fluid, the risk is that of neutralisation of varying amounts of the antibiotic. At the physiological stages of pharmacokinetics, there may be reduction of absorption and modification of urinary excretion. The most striking iatrogenic consequences result either from competition for binding to plasma albumins or from induction (or inhibition) of hepatic enzymes responsible for biotransformation (toxic overdose). Within bacteria, these interactions explain synergisms and antagonisms. They should lead to awareness of the multiple dangers of polypharmacy.