There is no universal scale for describing or measuring the severity of an adverse drug reaction. Assessment is largely subjective. Reactions can be described as mild, moderate, severe, or lethal.

Reactions usually described as mild and of minor significance include digestive disturbances, headaches, fatigue, vague muscle aches, malaise (a general feeling of illness or discomfort), and changes in sleep patterns. However, such reactions can be very distressing to people who experience them. As a result, people may be less willing to take their drug as instructed, and the goals of treatment may not be achieved.

Reactions that are usually described as mild are considered moderate if the person experiencing them considers them distinctly annoying, distressing, or intolerable. Other moderate reactions include skin rashes (especially if they are extensive and persistent), visual disturbances (especially in people who wear corrective lenses), muscle tremor, difficulty with urination (a common effect of many drugs in older men), any perceptible change in mood or mental function, and certain changes in blood components, such as a temporary, reversible decrease in the white blood cell count or in blood levels of some substances, such as glucose.

Mild or moderate adverse drug reactions do not necessarily mean that a drug must be discontinued, especially if no suitable alternative is available. However, doctors are likely to reevaluate the dose, frequency of use (number of doses a day), and timing of doses (for example, before or after meals; in the morning or at bedtime). Other drugs may be used to control the adverse drug reaction (for example, a stool softener to relieve constipation).

Severe reactions include those that may be life threatening (such as liver failure, abnormal heart rhythms, certain types of allergic reactions), that result in persistent or significant disability or hospitalization, and that cause a birth defect. Severe reactions are relatively rare. People who develop a severe reaction usually must stop using the drug and must be treated. However, doctors must sometimes continue giving high-risk drugs (for example, chemotherapy to patients with cancer or immunosuppressants to patients undergoing organ transplantation). Doctors use every possible means to control a severe adverse drug reaction.

Last full review/revision August 2007 by Joan B. Tarloff, PhD