Usually a powder, varying from white (pure) to brown when cut with various other substances to increase bulk (reducing strength) prior to sale.
Clinical heroin (Diamorphine) is pure, and normally comes in vials.

METHOD OF USE

Smoked.
Injected.
Snorted (not common).
Rectally (not common).

THE LAW

CLASS:  A
 
Possession:
Up to 7 years imprisonment and/or a fine.

Supply:
Up to life imprisonment and/or a fine.

EFFECTS/SIGNS OF USE

• Euphoria, drowsiness
• Involuntary leg movements, or shaking when needing a fix
• Needle marks on arms, legs, or other parts of the body
• Wraps of clingfilm or paper
• Blackened tinfoil
• Burnt or bent spoons
• Bloodstains on clothing or bedding
• Contracted pupils after using

DANGERS/RISKS

• Accidental overdose (purity is unknown when cut)
• Hepatitis or AIDS if shared injecting
• Blood infection & abscesses
• Malnutrition
• Wound botulism if injecting
• Substances used to cut the heroin can often be dangerous in themselves especially if injecting