If you are taking more medication than you can tolerate, you may feel groggy, or dizzy or sick to your stomach. You may have poor balance or see double at times. The dose at which these symptoms occur varies and some people can tolerate a much higher dose than others. In addition to side effects caused by the dose of medication, there may be symptoms that occur in some, but not all, people who take the drug. This is called an idiosyncratic side effect.
Some people notice weight gain, unrelated to calories eaten, on certain seizure medications.
Again, this does not happen to everyone, and is more likely to occur after prolonged use. Obviously, these symptoms may be caused by something other than epilepsy medicine, such as illness or stress. Give your doctor as much information as possible to help determine the underlying cause. This is one reason that your doctor may order more frequent lab work when you first start a new medication. Most of these serious side effects show up in the first few months you take a medicine, are not dose-related and are unlikely to occur later.