Like all drugs, epilepsy medicines have side effects. Some are dose-related, and become more likely as the dose increases. Sedation, slurring of speech, and unsteadiness are common effects of antiepileptic drugs at high doses. The condition is usually referred to as toxicity. Similar effects may occur at standard doses at the beginning of treatment and then slowly resolve as the body becomes accustomed to them. Alternatively, if clearance of the drug is compromised by age, illness, or drug interaction, a normal dose may slowly build up to a level which produces these effects.

Other side effects are associated with specific medications, and occur fairly frequently, regardless of dose. Depending on the drug involved, these effects may include double vision, weight gain, hyperactivity (in children), sleep disturbances, irritability, gum dysplasia, hirsutism, and changes in mood. In many cases, side effects occur at the beginning of treatment and may become less troublesome or even disappear as the body adjusts. Starting out at high doses may also increase side effects; these drugs are usually started at gradually increasing doses.

The most common side effects associated with epilepsy medicines are: drowsiness, irritability, nausea, rash, and clumsiness. Some drugs produce changes in emotions, memory or behavior, or affect learning. Occasionally, a drug will increase the number of seizures a person is having.

A third, rarer, type of side effect develops because of individual sensitivity or allergic reaction to a particular drug or drugs. This phenomenon may be termed an idiosyncratic reaction. Potentially fatal liver damage, aplastic anemia, and the severe rash of Stevens Johnson syndrome and related disorders are among the serious reaction group. While studies indicate an elevated risk for some populations and with some drugs (e.g., hepatic failure in young children on sodium valproate polytherapy; aplastic anemia in people taking felbamate; and an increased risk of severe rash in children on lamotrigine), most of the anticonvulsant drugs carry a slim but real risk of such reactions in people who are susceptible to them.

A prudent rule of thumb for parents and individuals on these medications is to be aware of the side effect profiles and to report changes in health, behavior, or mood to their attending physicians. Instances of prolonged fever, rashes, severe sore throat, mouth ulcers, easy bruising or pinpoint bleeding under the skin, weakness, listlessness, swollen glands, or lack of appetite may be signs that serious problems are developing and should be reported to the doctor at once.

None of the currently available antiepileptic drugs is proven safe in pregnancy; trimethadione and paramethadione are the only two known to be contra-indicated in pregnancy; both are older drugs which have now largely disappeared from use. Most of the front line established drugs have the potential to produce birth defects in a limited number of susceptible patients; polytherapy and elevated doses appear to increase the risk.

The most serious of these effects are cleft lip/cleft palate, cardiac abnormalities, and spinal tube defects (spina bifida). Taking folic acid prior to and during pregnancy has been recommended as a protection against spina bifida. A syndrome similar to fetal alcohol syndrome (wide set eyes, low set ears, short upper lip, slight abnormalities in fingers) has also been reported. Despite the elevated risk however, more than 90 percent of women with epilepsy who become pregnant give birth to normal, healthy babies.