Were you prescribed the liquid codeine by a doctor who also knows about or also prescribes the Lamictal? I personally do not think that it would hurt you, as I used to take Lamictal myself, and the codeine will not interact with it , as I have taken codeine and Lamictal at the same time before prescribed by the same doctor. It is always best to check with a doctor first though, because each person reacts to medicines differently.