Many physiological factors and conditions affect the way that drugs can affect a person's body. First, dosages are calibrated in terms of body weight; a normal dose of aspirin for an adult might kill a baby -- or a 100-pound ballerina would become very ill from the amount of alcohol a 250 pound football player could consume with almost no ill effects. Next gender affects how many drugs are metabolized, as oestrogen and testosterone interact with many types of medications. Age also affects drug tolerance, and a major current area of medical research is concerned with how drug doses and effects vary with age (an important issue for senior citizens who have been less well studied as a population than children). Heart conditions, diabetes, and other medical conditions may also affect how the body processes drugs. Also, drugs can interact with one another, and thus any chronic condition requiring medication can lead to potentially harmful interactions of ordinarily safe drugs with those being taken for a chronic condition.