The doctor tells Macbeth that his wife is very ill, and he cannot cure her. Macbeth reacts angrily, telling the doctor he cannot be bothered by such matters. This is so different than the start of the play when Macbeth and Lady Macbeth are shown as a close, loving couple. We know when Macbeth no longer cares about his wife that he is headed for tragedy.
The Doctor is concerned. (Remember, he has witnessed Lady Macbeth sleepwalking.) When Macbeth asks if there isn't something that the doctor can do- some drug he can give her - anything to make her become healthy again, the doctor replies that there is nothing he can do. He says that her illness is beyond his medicine for the physical body - she needs something for her soul (More needs she the divine than the physician) and that Lady Macbeth will have to help herself. Macbeth then says that if there is no medical cure, medicine is a useless profession ("Throw physics to the dogs...")
The doctor tells Macbeth that it is an ailment of the mind and that he (the doctor) or Macbeth can do nothing physically for her.
*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.