Writing a letter to a person who is in an alcohol or drug rehabilitation program can be an awkward experience if you're not sure what approach to take. Conversely, with a little guidance, the letters can serve as a reminder to the person in rehabilitation -- usually called "rehab" -- that he's loved and missed. This powerful message can give him emotional strength as he fights his addiction. Think of these letters as an opportunity to boost the patient's motivation and get him through lonely or difficult times.
Consult with the staff at the rehab facility first. They may have suggestions about what topics to avoid and the best approach to take. For example, they might encourage you to avoid giving advice, since you're not the patient's therapist and your advice may conflict with the advice given by the patient's actual therapist.
Avoid mentioning the patient's addiction in detail. Sometimes, people with addictions can be "triggered" by explicit talk of their addiction while in recovery. These emotional triggers can make them want to use alcohol or drugs again. Triggers go far beyond talk of the drug or object of addiction. They can also be set off when someone mentions places the person used to go to fulfil his addiction or the people he used to use alcohol or drugs with. These topics are something else you should be sure to consult with the staff before writing about.
Keep the conversation light and cheerful. It's likely that the person in rehab is undergoing strenuous and emotionally draining mental work to overcome his addiction. His therapy sessions may be very intense and adjusting to life in the facility and without his addictive substance will cause a lot of strain as well. Aim to make your letter serve as an emotional salve; it should be soothing and light. Avoid discussing sensitive or emotionally heavy subjects if at all possible.
Give the patient encouragement. Let him know you're proud of him and have faith in his ability to commit to this journey. If you have any motivational sayings near and dear to your heart, close with these. Above all, let the person you are writing to know that there's someone who cares about him who will be there for him through his journey. You will be effectively giving the person one more reason to fight his addiction, therefore enhancing his chance of success.