generally speaking,when(taken)according to the instructions,
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when后面所跟的动词的主语与主句主语一致.

when done:表示动作完成后发生主句描绘的情形,与主句主语是被动的关系

When asked by the police,he said that he remembered nothing .

when being done:与主句同时发生,主句主语与该动作是被动关系;

例如 when being laughed at,the boy smiled and walked away.

when doing:与主句同时发生,主动.

例如 when doing the exam,we should be careful

when to do 不适用于这种情况

when to do 中的不定式表示“什么时候要去做什么事”

例如 I don't know when to hand in my homework,

因此,在该句子中 “吃药”

drug 和 take 是被动关系,用taken