It is almost that patient vary widely in their beneficial responses to drug therapy which causes adverse reactions which is going to be a major public health problem. Many factors play a crucial role in occurrence of adverse reactions, some of these are patient related or drug related etc,,. Age for instance has a very critical impact on occurrence of adverse affects, alcohol intake also has an impact, and other factors like gender, race, pregnancy, breast feeding, kidney problem, liver function, drug dose and frequency. And some more are the following categories of differences among individuals are responsible for the variations in drug response: (i) individuals differ in pharmacokinetic handling of drugs attain varying plasma/target site concentration of the drug. This is more marked for drugs disposed by metabolism than for drugs excreted unchanged. (ii)Variations in number or state of receptors, coupling proteins or other components of response effectuation. A multitude of host and external factors influence drug response. They fall in two categories viz, genetic and nongenetic including all environmental, circumstantial and personal variables. Though individual variation cannot be totally accounted for by these factors, their understanding can guide the choice of appropriate drug and dose for an individual patient. So, the present study discuss about some of the factors and reasons that effect the drug response.