Story time. Once, I tried injecting in the dark so that I wouldn't awaken my roommate. When I aspirated, I obviously couldn't tell if I was in a vein or not, so I just went along with the injection and learned the hard way not to inject in the dark. I injected myself in the ass which felt like the gear was flowing through a vein, and it was. I immediately started coughing which grew to a rather extreme level within less than 30 seconds. I stumbled out of my room, coughing and sweating like there's no tomorrow. For 15 minutes, I coughed so hard I would start to gag. Luckily, nothing came out. My chest would hurt whenever I took in a deep breath for the next few days.

So yeah, actually. . . never inject anything into a vein for that matter.