MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Drug Effectiveness and Safety


Shalini S. Lynch

, PharmD, University of California San Francisco School of Pharmacy

Reviewed/Revised May 2022 | Modified Sep 2022

The main goals of drug development Drug Design and Development Many of the drugs in current use were discovered by experiments conducted in animals and humans. However, many drugs are now being designed with the specific disorder in view. Abnormal biochemical... read more are demonstrating the effectiveness and safety of the drug. Because all drugs Overview of Drugs A drug is defined by U.S. law as any substance (other than a food or device) intended for use in the diagnosis, cure, relief, treatment, or prevention of disease or intended to affect the structure... read more can harm as well as help, safety is relative. The difference between the usual effective dose and the dose that causes severe or life-threatening side effects is called the margin of safety. A wide margin of safety is desirable, but when treating a dangerous condition or when there are no other options, a narrow margin of safety often must be accepted. If a drug's usual effective dose is also toxic, doctors do not use the drug unless the situation is serious and there is no safer alternative.

The most useful drugs are effective and, for the most part, safe. Penicillin is such a drug. Except for people who are allergic to it, penicillin is virtually nontoxic, even in large doses. On the other hand, barbiturates, which were once commonly used as sleep aids, can interfere with breathing, dangerously lower blood pressure, and even cause death if taken in excess. Newer sleep aids such as temazepam and zolpidem have a wider margin of safety than barbiturates do.

Designing effective drugs with a wide margin of safety and few side effects cannot always be achieved. Consequently, some drugs must be used even though they have a very narrow margin of safety. For example, warfarin, one of the drugs that is taken to prevent blood clotting, can cause bleeding, but it is used when the need is so great that the risk must be tolerated. People who take warfarin need frequent checkups to see whether the drug is causing the blood to clot too much, too little, or appropriately.

Clozapine is another example. This drug often helps people with schizophrenia when all other drugs have proved ineffective. But clozapine has a serious side effect: It can decrease the production of white blood cells, which are needed to protect against infection. Because of this risk, people who take clozapine must have their blood tested frequently as long as they take the drug.

To help ensure that their treatment plan is as safe and effective as possible, people should keep their health care practitioners well informed about their medical history, drugs (including over-the-counter drugs) and dietary supplements Overview of Dietary Supplements Dietary supplements are used by about 75% of Americans. They are the most common therapies included among integrative medicine and health (IMH) and complementary and alternative medicine (CAM)... read more (including medicinal herbs) that they are currently taking, and any other relevant health information. In addition, they should not hesitate to ask a doctor, nurse, or pharmacist to explain the goals of treatment, the types of side effects and other problems that may develop, and the extent to which they can participate in the treatment plan.

Making the Most of Drug Treatment

People can help make their treatment plan as safe and effective as possible by telling the doctor, nurse, or pharmacist:

  • What medical problems they have

  • What drugs (prescription and nonprescription) and dietary supplements (including medicinal herbs) they have taken in the previous few weeks

  • Whether they have or have had any allergies or unusual reactions to drugs, foods, or other substances

  • Whether they follow special diets or have food restrictions

  • Whether they are pregnant, plan to become pregnant, or are breastfeeding

Additionally, people can help by

  • Knowing the brand name, generic name, or both of a drug and knowing what the drug is taken for

  • Reading the label on drug containers carefully before taking a drug, whether prescription or nonprescription

  • Understanding what a drug is being taken for, how to know whether the drug is working, and what side effects are possible

  • Knowing how long the drug should be taken

  • Not drinking alcohol if so advised

  • Not chewing, cutting, or crushing a capsule or tablet unless so instructed

  • Not using household spoons to measure liquid drugs

  • Knowing what to do if they miss a dose

  • Using simple tools like charts or medication organizers to remember to take doses at the correct times

  • Storing drugs in the correct place (cool, dry place; out of sunlight; and away from children and pets)

  • Disposing of expired drugs properly

  • Never taking someone else's prescription drugs

  • Taking recommended preventive steps and participating in recommended health programs

  • Keeping a drug list handy

  • Keeping appointments

  • Seeking medical care promptly when a problem develops

  • Contacting their provider or pharmacist with any questions that may arise

More Information

The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

quiz link

Test your knowledge

Take a Quiz!