And, of course, there isn't.

A medication doesn't alter your core personality; your personality constantly changes, adapts, to stimuli.  Why do some people go all Zimbardo at the drop of a hat, while men married of 20 years become infuriatingly repetitive in their behaviors?

It's no different than the antidepressant question, which is no different than anything else. You're confused because it's a medication, but there are other things more powerfully transformative than a pill. For example: divorce. You may think the "real you" married your wife or husband, but I am confident that if you had married someone else, you would have been a different person-- sometimes dramatically different. Example: a gazillion women who are in their second (happier) marriage have told me that they don't even recognize the woman that they were in the first marriage. "I can't believe the things I did-- in some ways, I was actually a bad person." And they describe being manipulated-- and manipulating; being selfish, etc. Circumstances made them "bad," and the healthier second marriage has made them into a healthier, better person.

Or whatever. I'm not saying all first marriages are bad, and you are bad for being in them-- I'm giving an example of how an external event that you chose drastically alters your identity.  The pill is no different.

The reason so many people can't accept this existential position is because many parts of personality seem to remain consistent despite significant events, meds, etc.  But much of the consistency has to do with consistencies in other environments. e.g. You had two marriages, but the same job; or you still saw your parents every weekend; etc, etc. Those anchors fix parts of your identity.

Friedman offers the example of a woman who has been taking Zoloft for 8 years, and has had a decreased libido.

She had understandably mistaken the side effect of the drug for her "normal" sexual desire and was shocked when I explained it: "And I thought it was just me!"

This is an exceptionally good example, because it shows why Friedman's logic is wrong. Why is it Zoloft's fault? How do you know she just doesn't have a low libido?

I know Zoloft has sexual side effects. But he jumps to the conclusion that because they have side effects, that must be why she has a low libido. But do sexual side effects happen in everyone? The package insert says 6-11%. I'll spot you 20 points. If 30% of patients have decreased libido, is it logical to blame her low libido on Zoloft, and not on anything else?  She doesn't say she has a decreased libido; she says she has a low libido. See the difference?

But even that explanation is a distraction.  Friedman misses the point altogether: she has a low libido. Period. Maybe it's the Zoloft, maybe it isn't, but unless she is coming off the Zoloft; unless she just started it that you are noticing a change from before; then the point is moot. Low libido is her identity.  Perception isn't reality, behavior is reality. 

What people want is there to be a core, perceived identity-- "I'm X"-- that can be pharmaceuticalized into existence without the requisite behavioral effort. "I should be happy." Well, actually, no. You shouldn't. You may want to be happy, but there shouldn't be an expectation of it.

If you are normally a happy person, and then become depressed, then you can say the meds are returning you to normal. But if you have always been depressed, and the meds have changed that, you are not returning to normal, you are moving further away from normal. I'm not going to judge this as good or bad-- I'm simply saying that you're not returning to core, you are different.

The key here is that early childhood if of huge importance in creating anchors, which will allow some consistency of personality throughout life.   Consequently, the extent to which you have some personal traits will determine how easily you change the others.  e.g. narcissists can be relied upon to become violent even when they "are" not violent people.

But, ultimately, you get to choose who you are. Choose.