Medicines need to be in the body at a certain concentration to work effectively. This concentration is not exact, but doctors, through clinical studies and historical data, typically know the lowest concentration and the highest concentration of the drug that do the job properly. Between these two limits is the therapeutic range, which can also refer to the dosages of the medicine the patient takes, rather than the concentration in the body. If a person takes too little medicine, and has a low concentration of the drug, then his or her medical problem may not be fixed or controlled. If the person takes too much medicine, then the risks of side effects may be unacceptably high.
Drugs are, by their nature, biologically active, and all carry a risk of side effects, some more serious than others. Too high a dose of medicine can increase the risk of side effects, and may also be potentially lethal. On the other hand, if a patient takes too little medicine, the medicine cannot act on the body efficiently enough to improve the medical condition, and the person may be at risk from the illness, or even of death.
Therefore, doctors assign a therapeutic range to medicines. A packet of aspirin, a relatively innocuous drug, instructs the user to take a certain amount of tablets, each of which contain a certain dose of the drug. This dose is in the therapeutic range that scientists have deemed relatively safe for minor medical conditions such as headaches.
When medicines are stronger than over the counter medications, or when a patient requires aggressive treatment, the therapeutic range needs to be very defined. Data that scientists have collected over the years is taken into account when establishing therapeutic range, but new drugs and even different versions of drugs require more research into the appropriate therapeutic range for individual conditions, and perhaps even for individual people.
In these cases, a doctor may take samples from a patient to check that the medical condition is becoming better from a certain dosage of medication, and that it is present within a safe therapeutic range. Blood samples are a common way of testing for the drug, as medicines typically move around the body through the blood system. Plasma is one of the components of blood, along with blood cells, and as it is the liquid that carries the cells and drugs around, is often extracted from the blood sample for the drug testing.
Some drugs stay in the body for a long time, and doctors may also test to see if previous doses are still circulating before allowing a patient to take more medicine. People who have too little medicine in the body can take more drugs to boost the concentration. Even if the doctors can control the drug level in the body to within normally accepted levels, some people may still experience side effects from the drug, as individuals react differently to medication.