An effect that takes hold immediately; something whose efficacy is instantaneous or striking.
その薬は効果覿面で、飲んだ翌日には症状が楽になった。
The medicine worked immediately; my symptoms eased the next day.
新しい方法を試したところ、効果てきめんで作業時間が半分になった。
When I tried the new method, it was immediately effective and cut the work time in half.
宣伝を変えたら客足が戻り、効果覿面だった。
After changing the advertising, customers returned — the effect was immediate.