Translation guide
In Japanese, 'adverse effect' is most commonly expressed with 副作用 (side effect) for medicine, or 悪影響 (bad influence/negative effect) for general contexts. The choice depends on whether the effect is a direct, often physiological reaction (副作用) or a broader negative consequence (悪影響).
To refer to an unintended, usually harmful effect of a drug or medical treatment.
The standard term for 'side effect' in medical and pharmacological contexts. It can be used for both mild and severe reactions.
Side effects of this medicine include nausea.
If you experience severe adverse effects, please consult your doctor.
To describe a broad negative impact on something, such as health, environment, economy, or relationships.
To emphasize a harmful bodily reaction, often used in clinical or formal contexts.
A technical term used in clinical trials and medical reports for 'adverse event'. It is not used in everyday conversation.
Serious adverse events were reported during the clinical trial.
Also used for adverse reactions, though it is broader. In medical contexts, it is the most common term.
To express a serious, often long-term negative consequence.
Literally 'harmful effect'. It is stronger than 悪影響 and often used in environmental or health risk discussions.
Air pollution has adverse effects on the respiratory system.
副作用 is specifically for side effects of medicines or treatments. 悪影響 is a broader term for any negative influence. Using 副作用 for non-medical contexts can sound odd.
I get sleepy as a side effect of the medicine.
Excessive smartphone use has an adverse effect on the eyes.
The direct translation '逆効果' (ぎゃくこうか) means 'opposite effect' or 'counterproductive result', not 'adverse effect'. Avoid using it unless you mean something had the reverse of the intended effect.
The policy had an adverse effect on the economy.
Watching too much TV has an adverse effect on children.
Fever was observed as an adverse reaction to the vaccine.