There are a range of different offences relating to controlled drugs. The most common ones are possession with intent to supply, supplying drugs, importing drugs, cultivating cannabis and possession of drugs.
Drugs are split into different classes, A, B or C. The most harmful drugs are class A, and these include heroin and cocaine. Sentences for supplying or importing these are measured in numbers of years. Class B drugs, which include cannabis attract lower sentences but people involved in supplying or commercial level growing can still receive significant prison sentences. Class C drugs are the least serious.
Simple possession of drugs, especially cannabis, in small quantities will normally be dealt with by way of a warning from the police and will not require a court appearance. However it remains a criminal offence. If the police want to interview you about possessing drugs, in any quantities, you should always have a legal representative present to advise you.