Prescriptions: Complying with the Doctor's Orders

Medical Author: William Shiel, MD, FACP, FACR
Medical Editor: Jay W. Marks, M.D.

"Why do I have to follow the directions, doctor? And, why should I stay on my medicine when my symptoms have improved?"


The patient 's questions presented above are important ones because they deal with critical issues about taking medications. More specifically, these questions address how medications work and how they must be used in order for them to be effective and safe.

I will review aspects of these issues with the goal of explaining the reasons for the doctor's orders and why it is important to comply with, that is, to follow the directions for taking medicines. I will discuss the following concepts:

  1. basic principles of pharmacology (the handling of medicines by the body);

  2. how medications affect disease;

  3. antibiotics in particular, since they are so commonly prescribed;

  4. the timing of doses of medicines;

  5. the regularity with which medicines need to be taken in order to prevent disease; and

  6. why medicines must be taken exactly as directed by the doctor to minimize the possibility of toxicity and side effects.


After we consume medications, they are absorbed from the intestine and into the blood. The speed with which absorption occurs varies according to the chemical structure of the medicine as well as the composition of the tablet or capsule. The frequency with which medicines must be taken usually depends on how quickly they reach their peak blood levels and how quickly they are eliminated from the body (metabolized) by the liver and/or kidneys.

Medicines must be administered in such a way that the balance of absorption and metabolism maintains a specific level (concentration) in the blood. For each medicine, scientists have determined the optimal dose and frequency of dosing to maintain concentrations in the blood that are high enough to maximize beneficial effects but low enough to avoid toxic side effects. If the doses are too low or are taken too infrequently, the medicine may not be effective. If the doses are too high or are taken too frequently, toxic side effects may occur.

Medicines must be taken only for the purpose for which they are intended. Doctors refer to these purposes as "indications." A medicine that is taken for an indication other than what the doctor prescribes it for has little chance of providing any benefit but it still can cause harm due to toxic side effects.

There also are reasons that some people should not take some types of medicine, even if the medicine is good for the condition that they have. These reasons are called "contraindications." For example, people who are allergic to some medicines should not take related medicines, and some people may have a second, unrelated illness that some medicines can seriously worsen. For these reasons, and others, people should never take another person's medication without a doctor's specific instructions.

Effects of Medicines on Disease

Medicines have different purposes in treating illnesses, including:

  • quieting and stopping the illness;

  • keeping the illness inactive;

  • preventing new illnesses; and

  • treating the symptoms of illnesses (such as pain).

For each of these purposes the medicines must be taken according to the doctor's directions in order for the patient to benefit. If the medication is inappropriately taken or discontinued, illnesses can persist unnecessarily, inactive illness can become active again, complications of the illness can develop, or an illness that was being prevented may occur.


The effectiveness of antibiotics to kill bacteria in the body depends on continuous, adequate concentrations of the antibiotics in the blood. Only adequate concentrations of the antibiotic will kill the bacteria in the tissues that are infected (for example, the ears, sinuses, throat, lungs, skin, etc.). If the antibiotic is not taken in adequate doses or doses are not taken regularly, the bacteria can survive and multiply. The infection can then persist unnecessarily and complications of the infection can develop.

Also, if the entire prescribed course of antibiotics is not completed, the bacteria may only be reduced in numbers, not eradicated, and the infection may return in full force after the antibiotics are stopped. Another potentially serious response to an incomplete course of antibiotics is bacterial resistance. Bacterial resistance occurs when a small number of bacteria survive because the course of antibiotics was not completed. The bacteria that survive are likely to be resistant to the antibiotic, that is, they have developed ways to block the effect of the antibiotic. When the antibiotic is next used, for example, to treat the infection when it recurs, the antibiotic is no longer effective. In this situation, the next infection can be more serious and it may be necessary to change to other antibiotics to stop the infection.

Timing of Medications

Although the timing of medicines is often dictated by the need to maximize benefits and limit side effects, doctors also might specify particular timing of doses and length of treatment in order to avoid interactions with other medicines. In addition, certain procedures or tests may require special timing of the prescribed medicines. Finally, doctors may recommend that medicines be discontinued because they may interfere with certain procedures or tests. Not heeding the doctor's directions could lead to side effects from interactions with medications or complications from a procedure or test.

The Role of Medicines in Disease Prevention

Medicines are commonly used to prevent illnesses. For example, aspirin is used to prevent heart disease and stroke while folic acid is used to prevent birth defects. However, if these medications are not taken regularly according to the doctor's orders, they may not prevent the disease.

Other examples include:

  • Vaccinations in children and adults must be given at specifically designated times.

  • Persons traveling to countries with malaria must take medicines to prevent malaria beginning well in advance of arriving in the country (so that beneficial blood levels of the medicine are present right from the beginning of the trip). The anti-malarial medicines must be continued during and after the trip in order to fully protect from malaria.

  • Birth control pills must be taken regularly and in the proper sequence, when applicable, in order to prevent pregnancy.

Avoiding an Overdose and other Side Effects

Medications easily can become toxic or give rise to side effects if not taken as directed. Taking more than the recommended dose can greatly increase the risk of side effects with no additional benefit for the user. A perfect example is the common pain-reliever, acetaminophen (Tylenol), which in larger-than-recommended doses can cause serious liver damage but does not provide greater pain relief.

Side effects and/or an overdose also can occur when taking another person's medications since they often are prescribed based on an individual patient's age and weight (in addition to considerations such as allergy and underlying illnesses, as discussed previously).


The bottom line is that to obtain the maximum benefit from medicines and minimize the potential for side effects, patients must always follow exactly the doctor's instructions for taking a medicine and complete the full course of the medicine.

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Harrison''s Principles on Internal Medicine, 14th edition, 2006