The Doctor overhears enough of Lady Macbeth's talking in her sleep to understand that she and her husband were guilty of King Duncan's murder as well as other crimes. He tells the Gentlewoman who is eavesdropping with him, "Go to, go to. You have known what you should not." The Gentlewoman replies, "She has spoke what she should not, I am sure of that."
A little later in Act 5, when Macbeth asks, "How does your patient, doctor?" he replies, "Not so sick, my lord, / As she is troubled with thick-coming fancies / That keep her from her rest." He obviously thinks she is mentally ill. Macbeth responds in some of Shakespeare's most poignant lines which show that he is suffering from the same illness as his wife:
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 stuffed bosom of that perilous stuff
Which weighs upon the heart?
The Doctor replies, diplomatically, "Therein the patient must minister to himself." He is suggesting that the only cure for Lady Macbeth, as well as for her husband, would be a religious one involving full confession of their sins. The Doctor feels that there is nothing he can do to help either of them, and he wishes he had never been summoned. He says to himself in an aside:
Were I from Dunsinane away and clear,
Profit again should hardly draw me here.