There are many factors involved in how a medication reacts in the body. The action of a medication in the body can be altered by the individual’s body size, family traits, age, psychosocial issues, presence of disease, allergies and tolerance to a drug.

• Size. A smaller person may need less medication than a larger person. Smaller individuals may be given a lower dose for this reason. Increased body weight may necessitate a higher dose. Proportion of fat to lean body mass can influence the action and distribution of a drug throughout the body.
• Family Traits. How our bodies react to a drug can be affected by the traits we inherit from our parents. Members of some families have the same unusual reactions to drugs. Others may share the same allergies to drugs.
• Psychosocial Issues. Psychosocial reactions can affect how we react to drugs. Stress can slow absorption by diverting the blood flow from the intestines. Studies show that a resident with a negative attitude is less likely to have good results from a drug than a person with a positive attitude. This is known as the placebo effect. The mind has more control over what happens in our bodies than is currently understood.
• Gender and Basal Metabolic rate (BMR). Certain medications should not be handled by pregnant women because of the risk of birth defects. Various hormones can affect drug response. Testosterone and estrogen, the primary sex hormones, can cause different effects with medications. Basal metabolic rate (BMR) is the least amount of energy required for a person to be at rest and alert while fasting for at least eight hours. A person with a high BMR may metabolize and/or eliminate a drug more rapidly than a person with a normal or low BMR.
• Disease. Disease can affect how a drug is absorbed, used and excreted by the body. A disease that causes poor blood circulation, for example, could slow the absorption, metabolism and excretion of a drug. A disease as common as the flu can alter how the body reacts to a medication. Fever tends to increase an individual’s rate of metabolism. This could mean the drug is metabolized and excreted much more quickly than is normal.