At first the metabolism of the medication will be more efficient.  Thus this class of medications works great at first, but over time you need to raise the dosage, because the additional enzymes produced by your liver break down the drugs too quickly, so you don't get enough of the stuff you really need to your brain.  Does this seem familiar?  It might, as nicotine is also an enzyme inducer, and works the same way, more on that in a bit.

There are going to be a buttload of drug-drug interactions.  Some meds will like the extra enzymes, and will work better.  Other meds won't get along with them, and won't work as well.  However, better isn't always to your benefit, as you can suddenly find yourself with a toxic level of of medication in your system.  Be sure to check the PI sheets of all meds you take carefully, double check with your doctor and pharmacist, and triple check on the Check My Meds Drug-Drug Interactions page.

Because your liver is busy producing extra enzymes, it may slack off on other things.  Thus the need for the occasional blood work with some of these meds.  You also just can't go on any fad diet on your own, as you might seriously throw your glucose processing off track.  You're not in the realm of MAOI dietary restrictions, but you really do need to consult with your doctor before any radical changes to your diet.

These meds will be especially sensitive to things like if you've eaten or not, if you smoke or not, any booze you might drink and how often you drink it.   Being wacky anticonvulsants it all depends on the med and the circumstances if these activities will increase or decrease the amount of active ingredients in your system.  For instance, if you take Dilantin and you normally don't drink and then knock back a few one night, you'll find that the mix of booze and Dilantin will boost Dilantin's efficacy that one time.  However if you're a steady drinker the booze usually wins out in grabbing the processing capabilities of your liver, and Dilantin won't work as well for you as it does for the chronically sober.

And as for smoking, since nicotine is also an enzyme inducer, it will make some of these meds work better.  At first anyway.  Like any enzyme inducer the effects diminish over time, and you have to keep upping your dosage to get the same effects.

They also do a real number on birth control pills.  There's practically an industry built around figuring out the interactions of EIAEDs and oral contraceptives.  I realize that The Pill is used for more than just birth control, so it's not as simple as switching to condoms.  If any of these meds come up as an option and you use any sort of hormone supplement, it's time to make an appointment with the doctor in charge of the hormones.  While practically all psychiatric meds seem to affect girly hormones in our bodies (see, I know that guys have estrogen too), the EIAEDs (and Topamax for some reason) also slam any sort of girly pills you might take.