In the United States, companies that manufacture medicines are required to publish certain kinds of information about each product. This document is commonly known as a "package insert" because it is usually included with each package of the medicine.

You can also read these documents (also called "prescribing information") online. The U.S. manufacturer of Dilantin, Pfizer, publishes package inserts (in PDF format) for each form of the medicine:

Capsules: http://www.pfizer.com/download/uspi_dilantin_kapseals.pdf
Tablets: http://www.pfizer.com/download/uspi_dilantin_infatabs.pdf
Liquid: http://www.pfizer.com/download/uspi_dilantin125.pdf

Some of the information may differ in other countries.