Son Been Taking Tegretol For Years And Years. His Levels

Son been taking tegretol for years and years. His levels always exactly where they need to be. The last two blood test for his levels were low and zero. The only change is he has started drinking whey protein drinks. Lots of it. He did admit to missing a couple of doses, but since then, has been diligent about taking his meds, and went for another blood test level. It should zero level. Can the whey protein drinks be keeping the tegretol from being absorbed