because the chemicals in drugs sometimes have an effect on more than one bodily function, organ or system at a time! for example - some antipsychotics reduce the chemical overstimulation in the brain which cause hallucinations (auditory, olfactory, visual, tactile etc.) but also stimulate the salivary glands, causing drooling. like most things in life - you have to weigh up the costs/benefits of stuff. btw - you can take other medication to stop some side effects, but they of course have side effects of their own too!