Is it permissible to deal with drug addicts and traffickers due to the nature of work, or is it obligatory to keep away from them?

If your work is far from drugs, assisting in their sale, and financial dealings with their proceeds, and you are secure from being tempted by those who trade in them, then there is no harm in dealing with them while advising and admonishing them. However, it is preferable to seek another job to avoid suspicion and association with them.