Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Risk Factors for Adverse Drug Reactions


Daphne E. Smith Marsh

, PharmD, BC-ADM, CDCES, University of Illinois at Chicago College of Pharmacy

Reviewed/Revised Mar 2023

Hereditary factors make some people more susceptible to the toxic effects of certain drugs (including medications). Various genes have been identified that affect how the body responds to drugs. For example, differences in certain genes can affect metabolism of drugs in the liver, allowing drug levels to rise and increase the chances of an adverse drug reaction. However, testing for these differences is complex and is not yet used routinely in clinical practice.

How mind-body interactions, such as mental attitude, outlook, belief in self, and confidence in health care practitioners, influence adverse drug reactions remains largely unexplored.

Use of Several Drugs

Taking several medications, whether prescription or over-the-counter, contributes to the risk of having an adverse drug reaction (see Drug-Drug Interactions Drug-Drug Interactions The effect a drug has on a person may be different than expected because that drug interacts with Another drug the person is taking (drug-drug interaction) Food, beverages, or supplements the... read more ). The number and severity of adverse drug reactions increase disproportionately as the number of drugs taken increases. The use of alcohol, which is also a drug, also increases the risk. Asking a doctor or pharmacist to periodically review all the drugs a person is taking and to make appropriate adjustments can reduce the risk of an adverse drug reaction.


Infants and very young children are at high risk of adverse drug reactions because their capacity to metabolize drugs is not fully developed. For example, newborns cannot metabolize and eliminate the antibiotic chloramphenicol. Therefore, it is not commonly used. Newborns who are given the medication may develop gray baby syndrome, a serious and often fatal reaction. If tetracycline, another antibiotic, is given to infants and young children during the period when their teeth are being formed (up to about age 8), it may permanently discolor tooth enamel. Children under age 18 are at risk of Reye syndrome Reye Syndrome Reye syndrome is a very rare but life-threatening disorder that causes inflammation and swelling of the brain and impairment and loss of function of the liver. The cause of Reye syndrome is... read more if they are given aspirin while they have influenza or chickenpox.

Older people are at high risk of having an adverse drug reaction for several reasons (see Aging and Drugs Aging and Medications Medications, the most common medical intervention, are an important part of medical care for older people. Without medications, many older people would function less well or die at an earlier... read more Aging and Medications ). They are likely to have many health problems and thus to be taking several prescription and over-the-counter drugs Overview of Over-the-Counter Drugs Over-the-counter (OTC) drugs are those available without a prescription. OTC drugs enable people to relieve many annoying symptoms and to cure some diseases simply and without the cost of seeing... read more . Also, as people age, the liver is less able to metabolize many drugs, and the kidneys are less able to eliminate drugs from the body, increasing the risk of kidney damage by a drug and other adverse drug reactions. These age-related problems are often made worse by undernourishment and dehydration, which tend to become more common as people age.

Older people are also more sensitive to the effects of many drugs. For example, older people are more likely to experience light-headedness, loss of appetite, depression, confusion, and impaired coordination, putting them at risk of falling and fracturing a bone. Medications that can cause these reactions include many antihistamines, sleep aids, antianxiety drugs, antihypertensives, and antidepressants (see table Some Drugs Particularly Likely to Cause Problems in Older People Some Drugs Particularly Likely to Cause Problems in Older People Some Drugs Particularly Likely to Cause Problems in Older People ).

Pregnancy and Breastfeeding

Many drugs and medications—for example, antihypertensive drugs such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers (ARBs)—pose a risk to the health and normal development of a fetus. To the extent possible, pregnant women should not take any drugs, especially during the first trimester (see table Some Drugs That Can Cause Problems During Pregnancy) Some Drugs That Can Cause Problems During Pregnancy* Some Drugs That Can Cause Problems During Pregnancy* . However, for some medications, including ACE inhibitors and ARBs, risk increases during the second and third trimesters of pregnancy. Use of any prescription medications, over-the-counter drugs Overview of Over-the-Counter Drugs Over-the-counter (OTC) drugs are those available without a prescription. OTC drugs enable people to relieve many annoying symptoms and to cure some diseases simply and without the cost of seeing... read more , and dietary supplements (including medicinal herbs) 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 during pregnancy requires a doctor's supervision. Social drugs (alcohol and nicotine) and illicit drugs (cocaine and opioids such as heroin) also pose risks to the pregnancy and the fetus and therefore should be avoided.

Medications and medicinal herbs may be transmitted through breast milk to an infant (see Drugs Use During Breastfeeding Medication and Substance Use During Breastfeeding When women who are breastfeeding have to take a medication, they wonder whether they should stop breastfeeding. The answer depends on the following: How much of the medication passes into the... read more ). Some drugs should not be taken by women who are breastfeeding, whereas others can be taken but require a doctor's supervision. Some drugs do not usually harm the breastfed infant. However, women who are breastfeeding should consult with a health care practitioner before they take any drugs. Social and illicit drugs may harm a breastfeeding infant.

More Information

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

Drugs Mentioned In This Article

Generic Name Select Brand Names
Emtet-500, Panmycin, Sumycin
Anacin Adult Low Strength, Aspergum, Aspir-Low, Aspirtab , Aspir-Trin , Bayer Advanced Aspirin, Bayer Aspirin, Bayer Aspirin Extra Strength, Bayer Aspirin Plus, Bayer Aspirin Regimen, Bayer Children's Aspirin, Bayer Extra Strength, Bayer Extra Strength Plus, Bayer Genuine Aspirin, Bayer Low Dose Aspirin Regimen, Bayer Womens Aspirin , BeneHealth Aspirin, Bufferin, Bufferin Extra Strength, Bufferin Low Dose, DURLAZA, Easprin , Ecotrin, Ecotrin Low Strength, Genacote, Halfprin, MiniPrin, St. Joseph Adult Low Strength, St. Joseph Aspirin, VAZALORE, Zero Order Release Aspirin, ZORprin
Commit, Habitrol, Nicoderm CQ, NICOrelief , Nicorette, Nicotrol, Nicotrol NS
quiz link

Test your knowledge

Take a Quiz!