Phenytoin is used to treat partial seizures and primary generalized tonic-clonic seizures.
Often doctors find that medicines are useful for purposes other than the ones approved by the Food and Drug Administration (FDA). It is legal to prescribe medicines for these "off-label uses."
Phenytoin is commonly used off-label in the treatment of neuropathic pain syndromes, most notably in trigeminal neuralgia, a syndrome of facial pain. What are the most common side effects of phenytoin?
Most people who take phenytoin tolerate it very well with few or no complaints. The most common complaints include:
If these problems do not go away within several days, or are really bothersome, call the doctor, but never stop or reduce the medicine on your own. Sometimes the doctor can help with these side effects by changing the prescription:
People who have just started taking phenytoin (or who have just started taking a larger amount) should be careful during activities that might be dangerous, until they know whether they are having any side effects.
Approximately 1 in 10 people who take phenytoin have a red rash within the first few weeks of taking it. If this happens, tell your doctor or nurse immediately, as rarely, the rash may be the beginning of a serious problem. Be prepared to describe the rash and its location, and possibly even to come in for a quick visit, whether to the primary care doctor, or to the neurologist. A rash should never be ignored and often requires a change in the medication.
Some side effects only appear after a person has been using phenytoin for a number of months or years. They are usually, but not always, related to taking high doses. The most common ones affect appearance:
Overgrowth of the gums (gingival hyperplasia) is more common in children than in adults. It can be reduced by vigorous brushing, daily flossing, and regular visits to the dentist, who may recommend additional treatments. If phenytoin is stopped, the gum growth usually improves, though some people notice persistent issues.
Long-term use of phenytoin has also been found to cause weakening of the bones. Bone disease is even more likely if a combination of seizure medicines is used. People taking phenytoin should exercise, take calcium and vitamin D supplements, and eat foods rich in calcium. Some non-food sources of calcium, such as Tums and Os-Cal, can reduce the absorption of phenytoin. They should not be taken within about 2 hours after taking the phenytoin.
Long term use of phenytoin has also been associated with damage to the nerves in the legs. This usually goes unnoticed by patients, and may be seen by the doctor during routine reflex testing. Rarely, numbness, tingling or burning sensations could develop in the feet or legs with long term use of phenytoin.
Finally, chronic use of phenytoin has been associated with damage to the cerebellum, a posterior brain region which controls coordination. Again, most patients have no or mild symptoms of cerebellar disease, however, a minority might be slightly unsteady on their feet after years of high dose phenytoin therapy.
Serious reactions to phenytoin are very rare.
As with many other seizure medicines, there is a very small chance of developing a rash so serious that hospitalization may be required. (It has been estimated that serious rash occurs in 2 to 5 people out of each 10,000 who take phenytoin, although about 1 in 10 people develop a rash on phenytoin.). Though the risk is small, call the doctor immediately if you notice a rash, since in a few cases these rashes are fatal.
In rare cases, allergic reactions to phenytoin can result in damage to the liver or bone marrow. Your doctor will perform blood tests intermittently to look for early signs of this kind of damage.
Another rare reaction to phenytoin is disease of the lymph nodes. Make sure to tell your doctor if you notice fever, rash, and swollen, tender lymph glands. These usually are found in the neck, armpits, or groin.
Phenytoin (like other seizure medicines) should never be stopped without specific instructions from your doctor. Abruptly stopping phenytoin can lead to repetitive seizures (status epilepticus), a life-threatening condition. Additionally, sudden cessation of phenytoin can affect the other medications you may be taking, making them more or less effective, and better or less well tolerated. Your doctor may be able to prescribe a different medicine if phenytoin is causing unpleasant side effects.
People with liver disease and those who must take certain other types of medicines may need to be extra cautious about taking phenytoin. That is why it is so important to make sure the doctor knows about any liver disease and about every kind of medicine that is being taken. Most of the time, phenytoin can be taken successfully if the patient works with the doctor to determine the correct amount to take.
Phenytoin can be taken with other medications, though there are frequent interactions between phenytoin and other medicines.
Medicines that contain calcium, including some antacids, can prevent the body from absorbing phenytoin. They can be used, but not near the time of a dose of phenytoin. They should not be taken for a couple of hours before or after taking phenytoin.
Phenytoin makes birth control pills less effective, so the chances of becoming pregnant are greater. Women who use pills for birth control should talk to the doctor who prescribed them immediately upon starting phenytoin. The same is true for some other forms of birth control such as Depo-Provera or implants. Usually, all that is needed is to change to a pill with a higher estrogen content, or in the case of the injection or implant, to shorten the period of use from 12 weeks to 10. Phenytoin does not affect barrier types of birth control, like condoms, IUDs, and diaphragms.
Phenytoin also affects the way the body handles many other medicines. For instance, it reduces the levels of other seizure medicines, making increased seizures a possibility, including:
It also interferes with common medicines used for other conditions including:
In the United States, the Food and Drug Administration (FDA) assigns each medication to a Pregnancy Category according to whether it has been proven to be harmful in pregnancy. Phenytoin is listed in Pregnancy Category D. This means that there is a risk to the baby, but the benefits may outweigh the risk for some women.
In fact, the vast majority of women who use phenytoin during pregnancy have normal, healthy babies. Certain types of defects are increased when phenytoin is taken during the first 3 months of pregnancy, but they are still relatively uncommon. Some of these are growth delay, mental retardation, mild facial defects, and short fingers with small nails. The risk of defects is higher for women who take more than one seizure medicine. Women with a family history of birth defects also have a higher risk.
Women with epilepsy who are pregnant or thinking about becoming pregnant should talk to their doctor about their seizure medicines. Taking more than one seizure medicine may increase the risk of birth defects, so doctors sometimes gradually reduce the number or amount of seizure medicines taken by women planning for pregnancy. This is not done routinely, however, because it increases the risk of seizures. Some kinds of seizures can injure the baby, so women should not stop using seizure medicines or reduce the amount without the doctor’s guidance.
All women who are capable of becoming pregnant should take at least 0.4 mg (400 mcg) of the vitamin called folic acid every day because it helps to prevent specific birth defects called Neural Tube Defects. (The most well-known of these is spina bifida, in which the spinal cord is not completely enclosed.) Women with epilepsy should take between 1 and 4 mg of folic acid daily during their reproductive years. If the doctor thinks a woman is at especially high risk, the larger dose of folic acid—4 mg (4000 mcg) per day—may be recommended, beginning before the woman becomes pregnant.
About 20% to 35% of women have seizures more often during pregnancy because of changes in hormones, changes in the mother’s blood volume or changes in how their seizure medicine is handled by the body. It is helpful for the doctor to check the levels of medicine in the blood regularly during pregnancy so that the dosage can be adjusted as necessary.
Some babies born to mothers taking antiepileptic medications have had inadequate blood clotting within the first 24 hours after birth. It is often recommended that the mother be given 10 to 20 mg of vitamin K per day during the last month of pregnancy to prevent this problem.
About 10-20% of phenytoin in the mother’s blood passes through breast milk, however how much of this actually enters the baby’s bloodstream is not known. Further, how phenytoin affects the baby is unknown. That said, it is strongly felt by neurologists and epileptologists that the benefits of breastfeeding largely outweigh the risks of exposing the baby to antiepileptic medications, including phenytoin. As a matter of fact, the American Academy of Neurology and the American Epilepsy Society both recommend breastfeeding in women with epilepsy.