Phenytoin helps prevent some types of seizure activity. It is often used to aid in controlling nerve pain associated with some cancers and cancer treatments. Nerve pain causes a burning, tingling sensation. Phenytoin also may be ordered to control a rapid or irregular heart rate. Phenytoin may be given to stop uncontrolled seizures. It may be used during brain surgery to prevent seizure activity. Additional uses are under study.
Phenytoin works on areas of the brain to limit electrical discharges and stabilize cellular activity. Like many drugs that control seizures, it also has proven helpful in managing nerve pain.
The dose ordered depends on blood levels of the drug determined during routine monitoring. For pain, doctors usually order 200-500 mg per day, either at bedtime or in divided doses. Patients usually start on a low dose. Depending on the patient's response and drug blood levels, the dose may be increased. For seizures, patients are usually started at 100 mg, three times per day. Blood is drawn to check the level of phenytoin in seven to 10 days. The dose is adjusted accordingly. The doctor may prescribe a dose based on an older person's weight. A child's dose also is based on his or her weight.
It is very important that this drug be used exactly as directed. This medication should be taken at the same time every day. Patients should take a missed dose as soon as it is noted. But patients should not take two doses within four hours of each other. This medication should be stored in a dry place, not in the bathroom.
Patients should not suddenly stop taking this medication. The abrupt withdrawal of phenytoin could trigger seizures. Patients should not crush or break extended-release drugs. Chewable tablets should be chewed before swallowing. Other pills should be swallowed whole. Older adults may be more prone to adverse effects than younger people. Patients should not change brands without approval of the doctor.
Phenytoin should not be taken by patients who are allergic to this drug. People with slow heart rates, certain other heart conditions, or a flaking, open skin condition also should not take it. Phenytoin may be used cautiously for patients with asthma, allergies, limited kidney or liver function, heart disease, and blood disorders. It also should be used with caution in those with alcoholism, diabetes mellitus, lupus, poor thyroid function, or porphyria, a rare metabolic disorder. Pregnant women should discuss the risks and benefits of this medication with the doctor. It has been associated with birth defects and possibly cancer in children born to women taking the drug. Expectant mothers who are taking it to prevent seizures should not abruptly stop the drug. Those using it for pain control should discuss its continued use with the doctor. Patients on this drug should not breast feed.
Drowsiness is a common side effect of phenytoin. Patients should exercise caution when driving or operating machinery. Alcohol may increase drowsiness. Patients should not consume alcoholic beverages while taking this drug. Other, less frequent effects related to the central nervous system include an unsteady gait, slurred speech, confusion, and dizziness. Patients may experience depression, difficulty sleeping, nervousness, irritability, tremors, and numbness. Twitching, headache, mental-health problems, and more seizure activity may occur. This medication may also cause nausea and vomiting, stomach upset, diarrhea, constipation, and swollen gum tissue. Side effects also include a rash, hair loss (alopecia) or excessive hair growth, vision changes, uncontrolled eye movements, and inflammation of the surface of the eye. Patients may develop chest pain, swelling, fever, increase in weight, enlarged lips, or joint or muscle pain. Patients should practice good dental hygiene to decrease the risk of gum disease. With the doctor's approval, it may be taken with food to decrease stomach upset.
Phenytoin may produce changes in the normal makeup of the blood, including high blood sugar levels and anemia. It may trigger disorders of the lymphatic system and cause liver damage. If the liver is not able to properly break down phenytoin, it can produce toxic effects, even at small doses. Doctors typically assess kidney and liver function prior to ordering it. The tests are repeated at regular intervals. Patients should notify the doctor promptly of any side effects. If a skin rash develops, the doctor will instruct the patient how to taper off and stop the drug.
Many drugs interact with phenytoin and may increase or decrease its blood levels. Phenytoin may alter the effectiveness of other drugs. The list of interactions is long and varied. Drugs that interfere with phenytoin include anticoagulants (blood thinners), sulfa and other antibiotics, anti-fungal agents, drugs used to treat ulcers, methadone, anti-depressants, and disulfiram, which is used to treat alcoholism. It also interacts with corticosteroids, estrogen hormones, birth control pills and injections, drugs to treat hypoglycemia, asthma drugs, other anticonvulsants, lido-caine, heart medications, Parkinson's disease drugs, anti-inflammatory drugs, narcotic pain relievers, and anti-cancer drugs. Additionally, taking phenytoin with certain antidepressants may cause seizures in some patients.
Alcohol ingestion can interfere with maintaining proper blood levels of phenytoin. Patients should not drink alcoholic beverages while taking this medication. Antacids and calcium can lower the effectiveness of phenytoin. These drugs should be taken two to three hours apart from phenytoin. Tube feeding may decrease the amount of phenytoin absorbed. Patients should not give tube feedings for two hours before and after taking this drug. Patients should talk to the doctor before taking folic acid. It may interfere with this drug.
Debra Wood, R.N.
—Disorder of the nervous system that causes seizures.
—a part of the immune system that includes lymph nodes and tissue.