side-effect

side-effect — noun

1. A negative physical reaction that a medicine, injection, or treatment causes bey

1.nounB2
Definition

A negative physical reaction that a medicine, injection, or treatment causes beyond the desired healing effect.

Example

Christopher stopped taking the painkillers because the side effects made him feel dizzy all day.

often plural: side effects

The clinic warned that a side effect of this injection is temporary skin redness.

Common collocations
  • experience side effects
  • cause side effects
  • mild side effects
  • serious side effects
  • common side effects
Synonyms
  • adverse reaction

    broader term covering any negative bodily response including allergies; more formal in register

  • aftereffect

    focuses on lingering effects that persist after treatment ends rather than concurrent effects

  • complication

    implies a more serious, often unexpected medical problem arising during a procedure

Usage Note

Frequently used in the plural form 'side effects' when listing or describing multiple reactions from a single course of treatment. Often paired with adjectives such as 'mild', 'serious', or 'rare'.

Common Mistakes

The drug has serious sides.
The drug has serious side effects.
In English, 'side' alone cannot replace the full compound noun 'side effect'.

2. An additional and often unintended consequence of an action, decision, or change

2.nounB2
Definition

An additional and often unintended consequence of an action, decision, or change, beyond the originally intended outcome.

Example

An unexpected side effect of the new road was that local shops gained more customers.

collocation: unexpected/welcome side effect of [situation]

Working from home had the welcome side effect of saving Asher money on transport.

Common collocations
  • unexpected side effect
  • welcome side effect
  • positive side effect
  • unintended side effect
Synonyms
  • byproduct

    more neutral in tone; does not imply whether the secondary result is good or bad

  • consequence

    broader and more general; can refer to the main result as well as secondary ones

  • repercussion

    often implies an indirect negative effect that spreads over time

Usage Note

Often modified by adjectives that describe the nature of the consequence, such as 'welcome', 'unexpected', 'unintended', or 'positive'. Unlike sense 1, this sense does not imply negativity.

Common Mistakes

A side effect of the new law was unexpected.
An unexpected side effect of the new law was...
The adjective should directly modify 'side effect' for natural English word order.

Synonym discussion

Sense: noun/1

While 'side effect' is the most common everyday term for a predictable, dose-related response to medication, 'adverse reaction' is broader and covers any negative bodily response including allergic reactions; it is more frequently used in formal medical contexts. 'Aftereffect' focuses on lingering symptoms that remain after treatment has stopped, such as fatigue lasting weeks after chemotherapy, rather than effects that occur during treatment. 'Complication' implies a more serious, unexpected medical problem that arises during a procedure or illness, such as an infection following surgery. For example, a doctor might say 'drowsiness is a common side effect of this antihistamine' but 'the patient developed a serious complication after the operation.' Understanding these distinctions helps learners choose the precise term for the type and timing of the unwanted medical response.

adverse reactionaftereffectcomplication ↗

Common collocations

Phrases

  • side effect of

Prepositions

Etymology

Old EnglishLatinFrenchEnglish

The compound 'side-effect' joins 'side' (from Old English 'sīde', originally 'flank', later extended to mean 'additional') with 'effect' (from Latin 'effectus', meaning 'accomplishment' or 'result', via French). It first appeared in medical writing in the late 19th century to describe the secondary bodily effects of drugs. By the early 20th century the term had spread beyond medicine into general use for any indirect consequence of an action.