The process of absorbing, digesting, and excreting a medicine or food is called metabolism. The way the body metabolizes a particular medicine affects how often it must be taken. It also determines whether it will interact with other medicines or be affected by conditions such as liver disease.

How fast the body absorbs Dilantin depends on what form is taken. When capsules are used, the amount of medicine in the blood will be highest after 4-12 hours (up to 24 hours if taken with food). If the tablets or liquid are taken, the same level will be reached in about 1½ to 3 hours. The total amount absorbed will be about the same, however.

Like many other medicines, Dilantin is broken down (digested) in the liver. People with liver disease must be cautious about taking it. Other people also need to be careful if they take other medicines that are digested in the liver, as many are. The metabolism of each medicine may be affected, changing how quickly it works and leaves the body.

This is why the doctor needs to know about everything that a person takes-prescription drugs, over-the-counter medications, vitamins and other dietary supplements, and herbs. In most cases, all the medicines can be used if the amounts are adjusted to allow for these changes.

When the amount of Dilantin in the blood gets fairly high (but still in the usual range for effective treatment), the liver may unable to get rid of it any faster if more medicine is taken. This means that a small increase in the dose may cause a large change in the level of Dilantin in the blood.