Police looking for woman who scurried out of South Station with somebody else's $40,000 violin

Sigh. A few things. Abusers of drugs are skinny not because of an effect of the drug, but because they tend to stop eating / eat crap sporadically. Speed is an upper - you tend to lose weight on uppers because they make you expend loads of energy will simultaneously surpressing appetite. Opiates are downers. They have zero effect on appetite but heavy abusers are skinny because they stop valuing eating food and just consume more drugs.

"Depression drugs", I assume you mean antidepressants, are almost exclusively not abuseable because they have no abuse potential. People gain weight on these because they are generally otherwise healthy, and SSRIs stimulate ones appetites. Theoretically, if an SSRI had abuse potential and could form addicts, they would be skinny for the above reason.

"Anxiety drugs", by which i assume you mean anxiolytics, are typically benzodiazepines. These have little effect on appetites alone. They have a high abuse potential, but benzo addicts are typically high functioning, a good way to understand this is to understand that benzodiazepines act on the brain the same way as alcohol. There are a tremendous number of alcoholics among us, high functioning in ever walk of life. There are no generalizations as to the weight of alcoholics. The last complication is abusers of benzos tend to be poly drug users.

It's complicated. But yes, you can be simultaneously morbidly obese and be shooting, snorting, plugging, or smoking heroin

Oh, one last thing. Heroin is most frequently snorted, not injected. Remember - heroin was created in a lab by a pharmaceutical company as a "less addictive form of morphine". Obviously that didn't work out. Heroin is LESS powerful than fentanyl, yet one is a legal prescription drug and the other is used by addicts.