There have been cases regarding individuals who are given a certain therapeutic drug to treat symptoms or to keep symptoms from occurring in which the individual has a very violent reaction to the drug or feels no affect whatsoever. Many times this happens because of the genetic makeup of the individual. The study of this phenomenon is called "pharmacogenomics" or "pharmacogenetics."
As an example, a woman had surgery to remove a tumor and was given codeine as a pain reliever. Although she was doing well after the surgery, as soon as she began treatment with codeine she developed a full-body rash, difficulty breathing, and an irregular heartbeat. When she was taken off the codeine, her reaction disappeared. Upon further study, it was found that she lacked the enzyme in her blood that metabolized (broke it down into different components) the codeine into morphine and other substances, so she was essentially being overdosed with codeine. The lack of the enzyme was directly related to a variation in the gene that produced it. This genetic variation is a polymorphism between normal individuals and those who carry it. Sometimes these polymorphisms can cause a very serious reaction in an individual that could lead to death.
In some cases, individuals "hypermetabolize" drugs. This occurs when there is too much of an enzyme present that breaks down the helpful drug too quickly, leading to a lack of response to the drug. This can happen when too many copies of the gene are made and too much enzyme is produced. In other cases, the special receptor that the drug binds to on cells or tissues is missing, again because of a variation in the gene that makes the receptor protein. When there is no receptor to bind the drug, the drug may not have any affect on the cells or tissues that it should.
Genetic testing to determine the polymorphisms that play a role in our response to a drug is typical of basic genetic analyses. DNA is removed from cells, manipulated to find a specific area on a specific chromosome, and compared to "normal" DNA. In this way, genetic variations can be seen that may play a role in the over- or under-responsiveness to a therapeutic drug. This testing can also determine an individual's resistance or sensitivity to the effectiveness of certain drugs used in viral therapy (HIV or hepatitis C drugs, for example).
There are many, many enzymes in our blood that act to metabolize or break down specific drugs, allowing them to be excreted in urine or by other means. At present, there are comprehensive testing programs in place that can give us an overall picture of our specific genetic variations that may cause us to be unresponsive or over-responsive to a therapeutic drug.
For more information on this subject, see our article Pharmacogenetic Tests.