Femitin Phenytoin Sodium Theophylline
ModeratePhenytoin speeds up how fast the body breaks down theophylline, so theophylline may not work as well when the two are taken together. The same effect is listed for a long run of other medicines, including HMG-CoA reductase inhibitors and TCAs, but for you the point is phenytoin with theophylline.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.