an adverse effect — unfavourable effect (formal/neutral)
collocationC1IELTS 7+formaloccasional
a negative or harmful result caused by something, especially used in formal or technical contexts such as health, policy, or science.
Say it like a native
Textbook The medication may engender deleterious adverse effects.
Native The drug can have side effects.
'Engender deleterious effects' is medical-formal; in speech people say 'side effects'.
Pattern: have/cause/produce/lead to an adverse effect (on something)
In use
- The new medication had an adverse effect on some patients, causing headaches and nausea.health
- While technology has brought many benefits, it can also have an adverse effect on young people's attention spans if not used responsibly.IELTS speaking
Common mistake
✗ I'm not adverse to the idea.
✓ I'm not averse to the idea.
'Averse' = reluctant; 'adverse' = harmful. Don't mix them up.
Common collocations
an adverse effect on— on, have, any, serious
Don't confuse it
'Adverse effect' is more formal and specific than 'bad effect' or 'negative effect', and is often used in contexts like health, policy, or scientific reports. It does not mean 'opposite effect' (which would be 'contrary effect').