Phenytoin is one of several medicines that can speed up how the body breaks down Tinidazole (listed here as T-zol). This is because phenytoin switches on the liver enzyme CYP3A4. When Tinidazole is broken down faster, less of it stays in the blood, so it may not work as well.
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.