Can be cut with various substances that can be harmful
A wrap of speed will only contain about 5-10% of actual Amphetamine
Tolerance can build up quite quickly
Prepared-for-injection can cause vein damage, ulcers and gangrene (deadening or decaying of body tissue especially the digits and limbs) and especially with dirty needles.
Taking a lot of speed can give your immune system a battering. You could get more colds, flu and sore throats if you use it a lot.
The Law
Amphetamine is a Class B Drug
If prepared for injection it becomes class A.
Possession can get you up to five years in jail and an unlimited fine. Supplying someone else, including your friends, can get you up to 14 years and an unlimited fine.