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When taking medicine, carefully follow the directions written on the bottle.
Literal
Medicine [obj-を] take-[when-とき][topic-は], bottle [on-に] written-[てある] directions [to-に] carefully follow-[command-なさい].
服用する is the formal register word for 'take medicine' — the everyday word is 飲む ('drink'), but 服用 is the term you see on packaging and in medical contexts. ~てある marks a resultant state from a deliberate action: 書いてある is 'has been written (and the result is still visible),' distinct from 書いている ('is writing' or 'has been writing'). 従う takes に, not を, for what is being followed.