Why does mrs dubose suffer from fits when jem and scout visit her?

Mrs. Dubose suffers from fits always, not only when Jem and Scout came. But, she suffers from fits because she was addicted to morphine, a strong painkiller. She had asked Jem to read to her because she wanted something to distract her from her addiction. But Atticus said later that she probably never heard a Word they told her because she was only thinking about getting her morphine pill, after Jem and Scout left.

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