必要な時に使う：as needed
"必要な時にだけこの薬をお飲みください"
- Please take this medicine as needed.
- Take this medicine as needed.
Only when you need this medisene,please
take it.
-> Please take this medicine only when you need it.
You should only take this medicine when you need it.
Only take this medicine as required.
この場合、次のような言い方ができますよ。
ーYou should only take this medicine when you need it.
「必要な時にだけこの薬を飲むようにしてください」
ーOnly take this medicine as required.
「必要に応じてこの薬を飲んでください」
ご参考まで！
アメリカ合衆国 
カナダ 
