Translation guide
The unintended secondary effect of a medication, treatment, or action. In Japanese, the most common word is 副作用, but other expressions exist for different contexts.
An unintended effect of a drug or medical treatment, usually negative.
An unintended consequence of any action or situation, not necessarily medical.
副作用 is the general term for side effects of drugs and treatments. 副反応 is specifically used for vaccine adverse events and is more technical. In everyday conversation, 副作用 is sufficient for most situations.
Do not translate 'side effect' literally as 側面効果 or サイドエフェクト. These are not natural Japanese. Use 副作用 or other expressions depending on context.
副作用はありますか？
Are there any side effects?
副作用が心配です。
I'm worried about the side effects.
The standard term for side effects of medication or medical procedures. Used in both formal and everyday contexts.
この薬の副作用には吐き気があります。
Side effects of this medicine include nausea.
副作用が強く出たので、医者に相談した。
I had strong side effects, so I consulted my doctor.
Often used specifically for vaccine adverse reactions. More common in medical or official contexts.
ワクチンの副反応として発熱が報告されています。
Fever has been reported as a side effect of the vaccine.
Also used metaphorically for unintended negative consequences of policies, decisions, etc.
その政策の副作用で、中小企業が苦しんでいる。
As a side effect of that policy, small businesses are suffering.
Refers to after-effects or repercussions, often of events or social phenomena. More like 'ripple effect' or 'aftermath'.
不況の余波で失業率が上がった。
As a side effect of the recession, unemployment rose.
Harmful effect or evil influence. Often used for negative side effects of systems or social changes.
急速な都市化の弊害が目立ってきた。
The harmful side effects of rapid urbanization have become noticeable.
Colloquial way to refer to an unintended extra, often used humorously for minor side effects. Literally means 'bonus' or 'freebie'.
Only use in very informal, joking contexts. Not appropriate for serious medical discussions.
この薬、効くけど眠くなるのがおまけでついてくる。
This medicine works, but drowsiness comes as a side effect (bonus).