The injunction “Do no harm” is hard to follow unless one knows what counts as harm, and there is no clear consensus about this issue. It does seem that by making a person feel worse, I am harming her. Feeling bad is in itself a bad thing, and it might also lead to other bad things. If I feel bad, then I may not be able to do other things that I would otherwise enjoy, things that I might believe have value in themselves. At the same time, it seems that I could be harmed if I am prevented from learning the truth about my situation. If I have false beliefs, I might make choices that I would otherwise not make, choices that lead me to feeling worse than I would otherwise have felt. Could I be harmed by being led to believe something false about myself even if this false belief never leads to any decrease of good feelings or any increase in feelings of pain, dissatisfaction, or discontent? Let’s imagine that I believe about myself that I am widely admired and deeply loved by my friends and family and that this belief gives me deep feelings of contentment and satisfaction. But let’s imagine also that I am completely deluded: I am ridiculed behind my back and privately despised by my friends and family who are hoping to achieve a big inheritance from me. Let’s suppose further that their secret is safe, unless you tell me the truth. Would I be made better off by learning the truth about myself?

But returning to your particular case. Even if one has figured out what counts as genuine harm, it's often a tricky matter in any particular situation to figure out which course of action will cause the least harm. For example, whether a given patient would be most benefitted were he to learn that his death is imminent (so that he could make wise decisions about what to do with the rest of his life), or whether he would be most benefitted by being "blissfully ignorant", will depend on the nature of the person and what choices he has. But in any case, most of us value knowing the truth about our situation, and even if we know that we tend to screw up our own lives and even if we believe that others could make better decisions for us, we still prefer to make informed decisions for ourselves. For all of these reasons, it has seemed to many that physicians should always disclose to their patients information about their medical condition (including, it would seem, what effect a given drug is having on the patient's health).