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.
この薬の副作用には吐き気があります。
Side effects of this medicine include nausea.
副作用が強く出た場合は、医師に相談してください。
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.
A general term for 'bad influence' or 'negative effect'. It is used in a wide range of contexts, from personal to societal.
騒音は健康に悪影響を及ぼす。
Noise has an adverse effect on health.
その政策は経済に悪影響を与えた。
The policy had an adverse effect on the economy.
A slightly more casual synonym for 悪影響. It is often used in everyday conversation.
テレビの見過ぎは子供に悪い影響を与える。
Watching too much TV has an adverse effect on children.
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.
治験中に重篤な有害事象が報告された。
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.
ワクチンの副反応として発熱が見られた。
Fever was observed as an adverse reaction to the vaccine.
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.
大気汚染は呼吸器系に有害な影響を及ぼす。
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.
薬の副作用で眠くなる。
I get sleepy as a side effect of the medicine.
スマホの使いすぎは目に悪影響だ。
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.