If you care for someone who has an issue with drugs or alcohol then you may want to try to use an intervention to encourage them to seek help. Interventions are a difficult thing to do well, but if they work then they can genuinely be life-saving.
A lot of people who are suffering from drug and alcohol addiction are in denial and feel that as long as they are functioning to some degree, everything is OK. This means that when they are approached they feel attacked. Interventions can help with this because they involve a larger group of loved ones, which takes away some of the "you are the only person who is saying this" issues.
If you want to run an intervention for someone then it can be beneficial to have a professional give advice on what to say, and how to say it. In addition, you should go into the intervention armed with information about clinics and rehab centers and with some clear requests. Tell your loved one that you are worried about them and that you would like them to seek help. Tell them that if they are willing to seek help then you will support them in any way that you can.
If you are dealing with someone who is at a critical stage of their addiction - a person who has started stealing, lying or covering up what they are doing, or who has reached the stage where the addiction is damaging their health, then you should make it clear that you are not willing to support them while they are like this. Do not enable them. In extreme cases, you may need to say that you are not willing to allow them to stay with you if they are putting their life at risk through drugs. Cutting off financial or practical support may be the only way to get through to them. This kind of ultimatum can be difficult and even painful to give, but in extreme cases of drug addiction where a person's life is at stake it may be necessary to make that ultimatum and to follow through with it if the person refuses to get help.
It is vital that the ultimatum is worded properly, to ensure that it is known that the person is coming from a place of love. They may react with anger, but you should know that it is the addiction that is talking, not the person themselves. Drugs can change the way a person thinks and feels, and can make a person do things that they know deep down they should not be doing.
Find a good rehab center in your area and then provide the person you are working with some support so that they can get the help they need. Be patient, but be on the look out for a relapse. It may take more than one try, but with help people can beat an addiction to drugs.