Repeated injections cause damage to the veins, thrombosis and abscesses. Sharing syringes can cause hepatitis and HIV infection. Clean injecting equipment is available from some chemists or needle exchange schemes. Cocaine and crack are very short acting. Cocaine or crack produces tolerance - users need to keep taking larger amounts to get the same effect.
High doses can cause overdose, regardless of how the drug is taken.
Cocaine and crack use may become compulsive.