One thing I don't think I made clear enough yesterday is that I'm not arguing that people shouldn't take drugs if they're depressed or ADD or OCD or what have you, because that is their "true" self. I am all in favor of better living through chemistry, and a quick glance through accounts of early twentieth century mental institutions should be enough to convince anyone who thinks that the mentally ill should just tough it out and learn to look on the bright side of life.
Rather, I am disputing the notion that there is a true self, either au natural or chemically enhanced. Both the pre- and post-medication selves regard themselves as the "true" self, with, I think, roughly equally valid claims. For that matter, both Megan McArdle two hours ago and Megan McArdle now regard(ed) themselves as my "true" self, even though they are slightly different. People don't need to justify their decision to feel better by saying that the self that feels better is closer to some sort of platonic ideal of me-ness. It's enough simply to want to feel better. The best justification for medication is that the medicated self wants to keep being that way, while the unmedicated self wishes it were fundamentally otherwise.