I suspect it is the core issues, psychology and influences behind the two drugs.
Vyvanse was prescribed. A medicine. There was no social scene involved with it. No enhancement of pleasure or acceptance. It was part of a health regimen with one purpose- to curb her eating which was, and is, her primary method of coping. Everytime she took it she was reminded of why she was taking it- cuz she's a pig. It was an intrusion into her life.
Meth, coke, whatever she's taking has a totally different connotation. She started to take it to gain acceptance into King Tut's harem of butter golems. It made her happy, energetic, and confident. She likes how it makes her feel. She gets "cool girl" acceptance vibes by Beezin'. And while Beezin' on drugs, she remains blissfully oblivious to all of her very many obvious problems. Which, is exactly what addictive substances do. The fact that it, like Vyvanse, is a stimulant and causes weight loss was an incidental.
It's the same logic she follows with everything in her life. If she has to be told to do something that she doesn't want to do, it's bad. If she comes to a decision on her own and it makes her feel good, even if it is detrimental, that's the road she takes. It's all about hedonism.
It's like the legendary hissy fit over the infamous Turkey Pepperette. That was BAD because it was suggested by a doctor. It has carcinogens because it's cured! But enough greasy fast food for 5 is just Beezin' (So fuck off). Which is where we are now.
I think this is accurate, but I'd also like to add a bit.
Psychiatric prescription stimulants -- Adderall, Vyvanse, Wellbutrin -- are engineered to
not get you high. In their least-processed forms, they will -- which is why things like sustained release capsules aren't just more convenient, they're much less prone to abuse. Of course you still feel some effect from them, but they're not competition for street drugs.
Chantal didn't like how Vyvanse made her feel for two reasons: First, she's a cool, hip, misunderstood gorl who would be crippled by antidepressants, or psych meds in general. It would take away her quirky spark, yaknowhatimean? She'd be just another zombie. Just another cog in the machine. Remember, she has the mind (at best) of a 15 year old, and this is how teenagers think.
Second, she didn't like that it stopped her bingeing. She wasn't getting the same dopamine hit from eating an entire pizza. Worse, she
couldn't eat an entire pizza even if she wanted to. She lost her one source of pleasure and her only means of coping -- with nothing to replace it. This was by design. It's how that drug works. That's why it's meant to be taken alongside therapy in order to stop bingeing -- it controls the physical while you work on the mental.
Coke/meth/whatever crack or crack-like substance she's smoking does impact her appetite (though not as much as one might think) but critically it also
replaces her addiction. She's still getting dopamine hits -- bigger and better, even!
tl;dr street drugs will actually get you high and will actually bring you some pleasure. Prescription drugs -- especially those designed to work on binge eating and other psychiatric issues -- don't. (Of course prescription drugs can be abused, but manufacturers go out of their way to make the class of drugs I'm talking about as un-fun as possible. Compared to a hit off a crack pipe, Vyvanse barely registers.)
