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Risk Factors for Adverse Drug Reactions


Daphne E. Smith Marsh

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

Last full review/revision Apr 2021| Content last modified Apr 2021
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Many factors can increase the likelihood of an adverse drug reaction (any unwanted effects of a drug). They include

Hereditary factors make some people more susceptible to the toxic effects of certain drugs. 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.

Certain diseases can alter drug absorption, metabolism, and elimination and the body's response to drugs (see Drug Interactions: Drug-Disease Interactions), increasing the risk of adverse drug reactions.

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 drugs, whether prescription or over-the-counter, contributes to the risk of having an adverse drug reaction (see Drug-Drug Interactions). The number and severity of adverse drug reactions increase disproportionately as the number of drugs taken increases. The use of alcohol, which is technically 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.

Did You Know...

  • In the United States, 3 to 7% of all hospitalizations are for treatment of adverse drug reactions.


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, is not commonly used. Newborns who are given the drug 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 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). They are likely to have many health problems and thus to be taking several prescription and over-the-counter drugs. 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. Drugs 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 and [1]).

Age reference

Pregnancy and Breastfeeding

Many drugs—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). However, for some drugs, including ACE inhibitors and ARBs, risk increases during the second and third trimesters of pregnancy. Use of any prescription drugs, over-the-counter drugs, and dietary supplements (including medicinal herbs) 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, therefore should be avoided.

Drugs and medicinal herbs may be transmitted through breast milk to an infant (see Drugs Use During Breastfeeding). 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 are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

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Research studies may use substances designed to resemble drugs, but that do not contain any active drug components. These may look exactly like a particular drug, but they are composed of starch, sugar, or other harmless, non-drug substances. Some people feel better after they take these. What is the term for this phenomenon? 
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