*All quotes are taken from the Norton Shakespeare, based on the Oxford Edition.
The Doctor believes that Lady Macbeth is suffering from troubles in her mind, which is keeping her from sleeping soundly: "Not so sick, my lord,/As she is troubled with thick-coming fanicies/That keep her from her rest." (V.iii.39-41)
Macbeth wants the Doctor to cure his wife. Macbeth believes that the doctor can administer some medicine to erase these troubling thoughts from Lady Macbeth's mind: "Cure her of that./Canst thou not minister to a mind diseased,/Pluck from the memory a rooted sorrow,/Raze out the written troubles of the brain,/And with some sweet oblivious antidote/Cleanse the fraught bosom of that perilous stuff/Which weighs upon her heart?" (42-47)
The Doctor replies that Lady Macbeth will have to erase these thoughts herself. Macbeth is obviously upset by this and says that the doctor is throwing medicine to the dogs.