Helping.

If someone you know has a drug problem and wants to stop you should do what you can to help. First, if they are willing to quit it makes it much easier. Then, find an appropriate reliable rehab center and check the person in. When he/she finally gets out you should help to make sure that person isn't going back to the drugs by spending time with him/her.
But, if there is someone who you think is addicted but doesn't want to admit it you should make them realize what they are doing. Motivate them to want to stop by telling life stories related to drugs. When you achieve this send him/her to a rehab center and continue to give your support.