What happens during the interaction between Macbeth and the Doctor?

MACBETH How does your patient, doctor?

DOCTOR Not so sick, my lord, As she is troubled with thick-coming fancies That keep her from her rest.


MACBETH 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?

DOCTOR Therein the patient Must minister to himself.

MACBETH Throw physic to the dogs. I’ll none of it.
Macbeth knows quite well what is the matter with his wife because the same thing is the matter with him. They are both plagued by guilt, remorse, pity, sorrow, and the resulting insomnia. If she can’t sleep, he can’t sleep either. Macbeth is being slightly sarcastic with the doctor, and the doctor knows it. Macbeth is implying that the doctor doesn’t really know medicine. This is true enough. Doctors in those dark days knew little about causes and cures. They had to do a lot of pretending. This doctor has at least had experience with all kinds of people. He understands that Macbeth is talking on several different levels of meaning. Macbeth is expressing concern about his wife, but he is really concerned about himself. The doctor’s reply is intentionally ambiguous. He does not say the patient must minister to herself but that the patient must minister to himself, letting Macbeth take it either way he wants—that the doctor is advising him to heal himself or that people in general must heal themselves when they are afflicted with “a mind diseased” and not with a physiological ailment.
Macbeth counters the doctor’s sage advice by expressing the contempt for medicine implicit throughout his long question. He says, “throw physic to the dogs.” This is his way of telling the doctor that his wife may be plagued by guilt and remorse, but he has no such problems himself. In other words, he is lying when he says he was only asking about his wife and not about himself.
All of us, if we live long enough, will be troubled with painful memories, although hopefully not as terrible as those of Macbeth and his wife. Looking back over our past, we are sure to regret some of the actions we took and others we failed to take. Somerset Maugham wrote,
What makes old age hard to bear is not the failing of one’s faculties, mental and physical, but the burden of one’s memories.
No one can pluck from our memory a single rooted sorrow. We have to live with them. Shakespeare expresses a similar idea in "Sonnet 30."
When to the sessions of sweet silent thought I summon up remembrance of things past, I sigh the lack of many a thing I sought, And with old woes new wail my dear time’s waste: Then can I drown an eye, unused to flow, For precious friends hid in death’s dateless night, And weep afresh love’s long since cancelled woe, And moan the expense of many a vanished sight. Then can I grieve at grievances foregone, And heavily from woe to woe tell o’er The sad account of fore-bemoaned moan, Which I new pay as if not paid before. But if the while I think on thee, dear friend, All losses are restored, and sorrows end.
The sonnet ends on an up-note, but this does not mean all his losses are permanently restored or all his sorrows permanently end. The poet is suggesting that we think about something else, which is pretty much what we do anyway. In fact, that seems to be what Macbeth is doing and has been doing ever since he murdered King Duncan. Perhaps all Macbeth’s acts of tyranny have been his way of keeping his mind off his guilt, remorse, and shame.