I have been on phenytoin for 29 years and the only time it has gone toxic is if it is taken with a cold or flu medication containing ANTIHISTIMINE or DECONGESTANT.
These drugs weaken the stomach lining causing the phenytoin to metabilize faster and causing more of the to enter the blood stream.
No "over the counter" medications containing Antihistimine should not be taken, most Doctors don't tell you that.
Also, soft drinks (diet) containing "ASPERTAME" can cause your blood levels to go toxic.
Diagnosed with Epilepsy & Ulcerative Colitis.
Current Medications: Diantin 275mg, Pheonobarb 150mg, Asocol 3,000 mg.