In 2002, I started having anxiety attacks. Their most potent trigger was drugs. I had my first one after combining Wellbutrin, Ritalin, codeine, Benadryl, and marijuana. My second one was from yohimbine. Then a caffeine pill. I finally figured out that the drugs were triggering these things after the fourth attack (from smoking pot). After that, I would sometimes have spontaneous attacks without any drugs, but drugs were the best trigger.
Why did the drugs start to have this effect on me? Before 2002, I was convinced that I could take any amount of any mind-altering drug, short of a physically toxic dose, without ill effects. I didn't respect people who "freaked out" on hallucinogens. I had taken insane combinations, like 700mg of dextromethorphan, 500mg of diphenhydramine, and as many balloons of nitrous as I could stay conscious for. At worst, I would have some restless energy toward the end of the trip.
I still have no idea what changed biologically. But subjectively, I realized that one cause of my anxiety was an increase in my mental rigidity. As a drug starts to kick in, it puts pressure on your mind-state on many levels. In the old days, before anxiety, coming up was a great feeling, and I would melt into it. In a lot of ways, I wouldn't even be the same person while tripping. The entire framework of my mind could be pulled fluidly by the drug.
When I was 19, my mind started resisting the pull. I don't know why exactly, but it was related to my social development. I feel like I have a lot more to say about this, but it's not coming to me now.