There are a number of reasons. Some drugs are given in doses determined by the size of the patient. Thus, a person who has greater weight would take a larger dose. Some drugs are given in amounts determined by age, In many cases, a child will be given a lower dose than an adult. Drugs may be given in amounts related to the severity of the illness, the more severe the illness, the greater the dose. Finally, individuals often react to drugs in different ways. One patient may get the necessary result with a small dose while another, for some reason that is not always understandable, will require a greater dose.