… is to have some personal experience solidly behind it.
Preceptor: Oooh, this patient is on Celexa and Lexapro?
Me: Well, that’s not good.
Classmate: Why?
Me: Because the active drug is the same – citalopram – so the effective dose is doubled.
Classmate: But it’s not the same dose…
Me: Right, but since Lexapro is escitalopram – the (S)-isomer only, whereas the same dose of Celexa contains both – it’s roughly twice as powerful, because the (R)-isomer isn’t metabolically active. So if the patient’s on 40mg of Celexa and 20mg of Lexapro, that’s equivalent to 80 mg of Celexa – which is actually TWICE the maximum dose for any therapeutic benefit.
Classmate: … Huh.
Me: That’s the theory, anyway.
Preceptor: That’s.. wow, that’s almost right.
Me: Well, I guess I should add that, #1) the explanation is slightly more complex than just the isomer ratio, and #2) there’s some evidence that the (R)-isomer is actually somewhat active – just less predictable, which is what Forrest Pharmeceuticals is clinging to to convince psychiatrists to continue to prescribe Lexapro over Celexa, despite the fact that Celexa has several generics while Lexapro has none.
Preceptor/Classmate: …
Alternatively, I guess you’d be more likely to honor your psych rotation by not being a smart-ass know-it-all. Or at least by not keeping people guessing about why you just happen to be such a smart-ass know-it-all about psych meds specifically.
But I’ve gotta say, that sounds like less fun.
(My evaluation is going to be interesting!)
Action Potential 
Well, if you change your mind about being a doctor, it sounds like you’re all set to start work as a drug rep.
Lexapro has a generic available now! We’re surprised Forrest hasn’t gone the Pfizer route and flooded the market with coupons to get people to stay with the brand.
WHAAAAAAT.
Oh my god, well, that’s some life-changing news. I love Forrest for coming up with this SSRI, but $35 a month adds up.