New friends. Hanging out with someone who will act on self destructive impulses and cannot recognize a really bad idea without trying it, will at some point make you the guinea pig for one of those stupid ideas and get a laugh when you have to be cut out of your wrecked car or you have to go to the er and get your stomach pumped .
IV drug use. Iv drug use is a serious health hazard, including skin bacteria (like staph) causing infectious endocarditis, hepatitis c and hiv. Bath salt is apparently a stimulant. Which can cause hallucinations, delusions, poor memory, poor concentration, and dilated pupil. Please suggest your friend to seek professional help so he can stop.
Infection phlebitis. He will develop a phlebitis which is an inflammation of the vein he injected the bath salts into. He may get an infection, either bacterial or, since he used a needle that had already been used before, viral like hepatitis b or aids. If he injected enough, he could cause himself serious heart and lung problems.