she must take an aspirin.句中的吃为什么不用eat

吃药，就是take medicine，这是惯用法。
如：
Even though I take medicine, I feel worse than I do yesterday.
虽然吃了药，却好像比昨天更严重。
He stayed with his daughter as she went into hospital and talked her down after she took medicine.

“服药”要用take，不能用eat，例如 take the medicine(服药)。

因为aspirin是一种药,吃药固定用take