Everyone responds to drugs differently. The way a person responds to a drug is affected by many factors, including genetic makeup, age, body size, the use of other drugs and dietary supplements (such as medicinal herbs—see see Overview of Medicinal Herbs and Nutraceuticals), the consumption of food (including beverages), the presence of diseases (such as kidney or liver disease), storage of the drug (whether the drug was stored too long or in the wrong environment), and the development of tolerance and resistance. For example, a large person generally needs more of a drug than a smaller person needs for the same effect. Whether people take a drug as instructed (see see Adherence to Drug Treatment) also affects their response to it. These factors may affect how the body absorbs the drug (see see Drug Absorption), how the body breaks down (metabolizes—see see Drug Metabolism) and eliminates the drug (see see Drug Elimination), or what effects the drug has on the body.
Because so many factors affect drug response, doctors must choose a drug appropriate for each person and must adjust the dose carefully. This process is more complex if the person takes other drugs and has other diseases, because drug-drug and drug-disease interactions are possible.
A standard or average dose is determined for every new drug. But the concept of an average dose can be like "one size fits all" in clothing: It may fit a range of people well enough, but it may fit almost no one perfectly. For some drugs, however, the dose does not have to be adjusted, because the same dose works well in virtually everyone.
Effects of age:
Infants and older people particularly have problems with drug response. Their liver and kidneys function less effectively, so drugs that are broken down by the liver or excreted by the kidneys tend to accumulate, thus potentially causing problems.
Older people typically have more disorders than children and younger adults and thus usually take more drugs (see Aging and Drugs). The more drugs people take, the more likely they are to have problems caused by one drug interfering with another drug or disease. With aging, people also may have more difficulty following complicated instructions for taking drugs, such as to take the drug at very specific times or to avoid certain foods.
Last full review/revision July 2013 by Daniel A. Hussar, PhD