Many people are taking many prescription drugs. These drugs can improve the quality of life or assist in keeping a person healthy and alive.

Frequently, there are no adverse effects of using drugs, but on occasion there are difficulties. One such problem is an allergy to the drug. This may manifest itself as a skin rash. A more serious problem arises if the drug is toxic and may cause damage to the liver or kidney, for example. A life-threatening situation occurs if the person should go into anaphylactic shock with difficulty breathing and low blood pressure. If a reaction to a drug occurs, this must always be recorded in order to alert health care professionals of the potential risk in the future.

Because patients frequently take multiple drugs, one must keep in mind that interactions between drugs may affect dosages. The resulting interactions can lower or raise the drug level and thus be less efficacious or more toxic.

A drug is given in response to a complaint by a patient or when a test reveals an abnormality for which a drug can be beneficial. As drug costs continue to increase as well as the potential health risks to patients, proper prescribing becomes increasingly important. This will require improved testing to define given disease states as well as clinical studies to evaluate drugs for their efficacy in the treatment of specific diseases.

There are times when there are suggestions that a disease process exists without hard evidence to state unequivocally that the disease is present. Under these conditions some are willing to use empirically a drug and to monitor its efficacy to treat the condition. If the health of the patient improves while keeping all other parameters constant, one could surmise that the drug improved the condition. If there is no improvement, justification for continued use may be lacking.

There are several factors that may affect drug levels. First is the method of administration. If it is given orally, foods, diseases of the gastrointestinal tract or previous surgical removal of a portion of the tract may affect absorption and the subsequent drug level in the blood.

Once the drug is absorbed into the blood system, it will be metabolized. If the liver metabolizes a drug and the liver is diseased, the drug level may be elevated because of impaired metabolic function.

After the drug has been in the blood system, it and its metabolites or breakdown products will be excreted. If the kidney excretes it and the kidney cannot perform this function normally, the drug level in the blood may be elevated.

Some drugs are stored in the body once they have entered the blood system. That is, they leave the blood system but may reside in another compartment such as fat or lipids. Then later they will come out of the fat and into the blood system. This may affect the blood level of the drug.

In evaluating a drug, it is necessary to determine that an adequate concentration of the drug arrives to the target organ and not just in the blood system. For example, if the target organ is the brain, the level in the blood should reflect the drug concentration in the brain itself. Sometimes a so-called blood-brain barrier exists whereby the drug may be hindered from getting into the brain where it does its work.

The elimination half-life of a drug is the time it takes for half of the drug to be removed from the patient's blood level once the drug has reached equilibrium. If one uses the maintenance dose at the start of using a drug, about 5 half lives of the drug are required for the blood level to reach equilibrium in the blood. For example, if the half-life of a drug is 8 hours and the maintenance dose is 200 mg for the average patient, then it will take 40 hours for the drug concentration in the blood to reach equilibrium or steady state.

Therapeutic drug monitoring can be used to assist in the determining the doses of drugs to insure an adequate blood level to treat successfully the disease for which the drug was prescribed. In addition, such monitoring can be used to prevent the level of the drug from reaching toxic levels and cause problems for the patient. The use of peak and trough drug levels from a patient during a given time interval can determine the half-life of the drug for that specific patient. This will take into consideration the patient's own absorption, metabolism, storage and excretion. Additional parameters such as height, weight, age, gender and other lab findings can be used to construct a multivariate analysis to customize the drug dosing for a particular patient.

The benefits of therapeutic drug monitoring should be explored in light of what the alternatives are to a patient in terms of impaired health if no monitoring is done. It does not make sense to treat patients with subtherapeutic doses of a drug proven to be efficacious for a disease. A misinterpretation may follow so that the treatment plan may be incorrectly changed to another drug or even another treatment modality that is less efficacious or more costly. On the other hand, having to treat a patient once toxicity has occurred is clearly more complicated.