In listening to my mother describe her best friend's struggles with her daughter who has Schizophrenia I am reminded of an important tenet I learned while working in the state hospital: Never Argue with a Delusion.
Delusions are when people believe something which is not true. They can manifest themselves in many, many different varieties. Paranoia can be one, where the client believes that people are watching them, conspiring against them, poisoning their food or medicine, or trying to hurt them. Grandiosity can be another version, where the client believes they have a million dollars in the bank, that they are a messenger sent by their God, or they are a very powerful or influential person. Delusions can also be quite mundane or simple. Clients can be believe that messages are coming to them through the radio or television, or that family members are lying to them about a certain issue.
Whatever flavor of delusion you are working with, you can <em>never </em>beat it, except with medication. Using logic, reason, or trying to show the person the reality of the situation ("look - I'll eat a bite of your lunch to show you that it is not poisoned") will never work. The client will always find away around it ("You haven't put the poison in yet" or "You knew I wouldn't eat it so you didn't poison <em>this</em> meal"). Trying to argue with a delusion accomplishes nothing except to frustrate the family, cause an argument between the family member and the client, and possible result in hurt feelings or even a violent confrontation.
The most important thing is to realize they are not doing this on purpose. They're not just being stubborn or obtuse and they're not doing it to get your goat. To address it, try to accept it if you can and focus on dealing with the issue of getting the medication adjusted to appropriate levels.