Mainlining is -
Most persistent drug abusers follow a classic progression from sniffing (similar to oral route) to skin popping (subcutaneous route), to mainlining (intravenous route) - each step bringing more intense experience, a higher addiction liability. With mainlining the initial 'thrill' is more intense.
Cocaine addiction
Homicidal cut wounds
Drug addicts using main veins for injecting drugs
Suicidal cut wounds