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.
The standard term for side effects of medication or medical procedures. Used in both formal and everyday contexts.
Side effects of this medicine include nausea.
I had strong side effects, so I consulted my doctor.
Often used specifically for vaccine adverse reactions. More common in medical or official contexts.
Fever has been reported as a side effect of the vaccine.
An unintended consequence of any action or situation, not necessarily medical.
Also used metaphorically for unintended negative consequences of policies, decisions, etc.
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'.
Harmful effect or evil influence. Often used for negative side effects of systems or social changes.
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.
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.
Are there any side effects?
I'm worried about the side effects.
As a side effect of the recession, unemployment rose.
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.
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).