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I think they're saying the kid is taking it to avoid bullying, not to hurt his parents.
Still, yeah that's a little specific.
Working on: Author Appeal | Sandbox | Troper WallNobody was questioning that? We just don't think it's a trope.
Working on: Author Appeal | Sandbox | Troper WallAh, you mean, the other kids are being cruel to this particular kid? Didn't think of that, erm. Anyway, substances with legitimate medical use can be abused and have (always) side effects (if someone's telling you a particular medicine doesn't, they're probably selling it). So: the situation itself is specific, but we do have Pharmacology Tropes - we also have Weight Woe (which the kid in question is suffering from) and Bullying the Disabled (if the kid is so obese it affects his life in ways besides bullying, it may count). Weight Loss Horror is more of what I had in mind - see if it fits.

Because Kids Are Cruel, a clearly obese 10-year-old takes weight-loss medication without parental knowledge.