If you get into trouble with the police for taking or supplying drugs, you can get advice from Release (www.release.org.uk). They currently have a helpline on 0845 4500 215 (although government cuts has put this in doubt).
If you are worried about a drug problem and want to break the habit, contact Narcotics Anonymous (www.ukna.org) or contact their helpline on 0300 999 1212.
If alcohol is the problem, you could approach Alcoholics Anonymous (www.alcoholics-anonymous.org.uk); it also runs a helpline 0845 769 7555.