I think what you are referring to in drug A that is so beneficial is the removal of irrational fears. In other words, you have these emotions that you know aren't inline with your preferences, therefore, it would only be rational to have a drug that only removes those emotions.

But what if drug A completely changes your preference on something. Say, before you were pretty happy working towards your thesis, and working at starbucks, but after taking drug A you suddenly really start caring for people.

You end up wanting to quit everything and join some charitable group. Let's assume that you may even get more bliss when you start doing charitable work, I still believe you wouldn't choose to take the drug. You wouldn't knowingly change your preferences to something else even if it makes you happier. You wouldn't be optimizing your current preferences by taking the drug.