The purpose of using drugs is to relieve symptoms, treat infection, reduce the risk of future disease, and destroy selected cells such as in the chemotherapeutic treatment of cancer. The best treatment, however, may not require a drug at all. Recognizing that no effective medication exists is just as important as knowing which one to select. When more than one drug is useful, physicians select the one that is most effective and least hazardous. A recently developed drug may promise better results, yet it will be less predictable and possibly more expensive.

Every drug has multiple actions: it will affect organs and systems beyond those to which it is specifically targeted. Some patients may also experience idiosyncratic effects (abnormal reactions peculiar to that individual) as well as allergic reactions to certain drugs—additional reasons to select drugs carefully and avoid their use altogether when simpler measures will work. A case in point is the belief that penicillin or other antibiotics will cure viral infections—they will not. While new antiviral drugs are under development, using antibiotics unnecessarily is unwise and potentially dangerous. The number of drug-resistant organisms is growing and must be counteracted by the judicious prescribing of these chemicals.

Unnecessary drug use also increases the possibility of drug interactions that may interfere with drug effectiveness. Interaction can occur in the stomach or intestinal tract where the presence of one drug may interfere with the absorption of another. Antacids, for example, reduce the absorption of the antibiotic tetracycline by forming insoluble complexes. Of greater importance is the interference of one drug with another. Some drugs can inhibit the metabolism of another drug, which allows the amount of the drug to accumulate in the system, leading to potential toxicity if the dose is not decreased. Cimetidine, a drug used to treat peptic ulcers, causes few side effects by itself, but it does inhibit drug-metabolizing microsomal enzymes in the liver, increasing concentrations of many drugs that depend on these enzymes to be metabolized. This inhibition can be serious if the other drug is the anticoagulant warfarin. Bleeding can result if the dose is not reduced. Many other drugs are affected, such as antihypertensives (e.g., calcium channel blockers), antiarrhythmics (e.g., quinidine), and anticonvulsants (e.g., phenytoin). One drug can also decrease the renal excretion of another. Sometimes this effect is used to advantage, as, for example, when probenecid is given with penicillin to decrease its removal and thereby increase its concentration in the blood. But this type of interaction can be deadly: quinidine, for instance, can reduce the clearance of digoxin, a drug used to treat heart failure, potentially increasing its concentration to dangerous levels. Two drugs can also have additive effects, leading to toxicity, though either one alone would be therapeutic.

Problems with drug interactions can occur when a patient is being treated by different physicians and one physician is not aware of the drug(s) that another has prescribed. Sometimes a physician may prescribe a drug to treat a symptom that actually is a side effect of another drug. Discontinuing the first drug is preferable to adding another that may have side effects of its own. When a new symptom occurs, a recently initiated drug usually is suspected before other causes are investigated. Patients are advised to inform their physicians of any new drugs they are taking, as well as consult with the pharmacist about possible interactions that a nonprescription drug might have with a prescription drug already being taken. Having a personal physician who monitors all the drugs, both prescription and nonprescription, that the patient is taking is a wise course to follow.

In the United States, responsibility for assuring the safety and efficacy of prescription drugs is delegated to the Food and Drug Administration (FDA). This includes the approval of new drugs, identification of new indications, official labeling (to prevent unwarranted claims), surveillance of adverse drug reactions, and approval of methods of manufacture. Before an investigational new drug (IND) can be tested in humans, it must be submitted to and approved by the FDA. If clinical trials are successful, a new drug application (NDA) must be approved before it can be licensed and sold. This process usually takes years, but if the drug provides benefit to patients with life-threatening illnesses when existing treatments do not, then accelerated approval is possible. Physicians can receive permission to use an unapproved drug for a single patient. This consent, called emergency use and sometimes referred to as single-patient compassionate use, is granted if the situation is desperate and no other treatment is available. The FDA also sometimes grants approval to acquire drugs from other countries that are not available in the United States if a life-threatening situation seems to warrant this action. Another way to gain access to an investigational drug is to participate in a clinical trial. If it is a well-controlled, randomized, double-blind trial rather than an “open trial”—in which the investigator is not “blinded” and knows who is the subject and who is the control—the patient runs the risk of being given a placebo rather than the active drug. The Federal Trade Commission (FTC) has responsibility for “truth in advertising” to assure that false or misleading claims are not made about foods, over-the-counter drugs, or cosmetics.

Similar drug regulatory agencies exist in other countries and governing bodies as well. For example, China has its own Food and Drug Administration, which regulates drugs, medical devices, and cosmetics. In Europe the European Medicines Agency approves drugs, while also overseeing the scientific evaluation of medicines and monitoring the safety and effectiveness of drugs marketed within countries in the European Union.

A rare disease presents a unique problem in treatment because the number of patients with the disease is so small that it is not worthwhile for companies to go through the lengthy and expensive process required for approval and marketing. In the United States, drugs produced for such cases are made available under the Orphan Drug Act of 1983, which was intended to stimulate the development of drugs for rare diseases.

Controlled substances are drugs that foster dependence and have the potential for abuse. In the United States, the Drug Enforcement Administration (DEA) regulates their manufacture, prescribing, and dispensing. Controlled substances are divided into five classes, or schedules, based on their potential for abuse or physical and psychological dependence. Schedule I encompasses heroin and other drugs with a high potential for abuse and no accepted medical use in the United States. Schedule II drugs, including narcotics such as opium and cocaine and stimulants such as amphetamines, have a high potential for abuse and dependence. Schedule III includes those drugs such as certain stimulants, depressants, barbiturates, and preparations containing limited amounts of codeine that cause moderate dependence. Schedule IV contains drugs that have limited potential for abuse or dependence, and includes some sedatives, antianxiety agents, and nonnarcotic analgesics. Schedule V drugs have an even lower potential for abuse than do schedule IV substances. Some, such as cough medicines and antidiarrheal agents containing limited amounts of codeine, can be purchased without a prescription. Physicians must have a DEA registration number to prescribe any controlled substance. Special triplicate prescription forms are required in certain states for schedule II drugs, and a patient’s supply of these drugs cannot be replenished without a new prescription. Similar drug enforcement agencies exist in other countries and international regions as well, including the European Union, where illicit drug use and drug trafficking are policed by the European Drug Enforcement Agency.