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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Brand Name
Phenytoin
Generic Name
Phenytoin
Active Ingredient
PhenytoinCategory
Anti-epileptic Agent [EPC]
Salt Form
Sodium
Variants
10
Different strengths and dosage forms
| Strength | Form | Route | NDC |
|---|---|---|---|
| 50 mg/1 | TABLET, CHEWABLE | ORAL | 64980-573 |
| 50 mg/1 | TABLET, CHEWABLE | ORAL | 70518-0841 |
| 50 mg/1 | TABLET, CHEWABLE | ORAL | 51672-4146 |
This page is for informational purposes only and does not replace medical advice. Before using any prescription or over-the-counter medication for Phenytoin, you must consult a qualified healthcare professional.
| 125 mg/5mL | SUSPENSION | ORAL | 59762-0531 |
| 125 mg/5mL | SUSPENSION | ORAL | 0121-0892 |
| 50 mg/1 | TABLET, CHEWABLE | ORAL | 60687-156 |
| 125 mg/5mL | SUSPENSION | ORAL | 51672-4069 |
| 50 mg/1 | TABLET, CHEWABLE | ORAL | 0904-7199 |
| 125 mg/5mL | SUSPENSION | ORAL | 17856-4070 |
| 125 mg/5mL | SUSPENSION | ORAL | 60687-275 |
Detailed information about Phenytoin
Phenytoin is a potent hydantoin-derivative anticonvulsant used to control generalized tonic-clonic and complex partial seizures by stabilizing neuronal membranes. It is a high-alert medication requiring precise therapeutic drug monitoring.
For most adults, the goal of Phenytoin therapy is to achieve a steady-state serum concentration between 10 and 20 mcg/mL. Healthcare providers typically initiate therapy with a loading dose or a gradual titration.
Phenytoin is approved for use in children, but dosing is highly individualized based on weight and age.
In patients with significant kidney disease, the protein binding of Phenytoin is often reduced. This means a higher percentage of the drug remains "free" and active. While the total Phenytoin level might look low, the active level could be toxic. Doctors often use the "Sheiner-Tozer" formula to adjust and interpret levels in these patients.
Since Phenytoin is primarily cleared by the liver, patients with cirrhosis or hepatitis are at a high risk of drug accumulation. Lower starting doses and slower titration are mandatory to prevent toxicity.
Older adults often have lower albumin levels and decreased liver clearance. Clinical guidelines suggest starting at the lower end of the dosing range and monitoring for early signs of ataxia (unsteadiness) or confusion.
Consistency is the most important factor when taking Phenytoin.
If you miss a dose, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and return to your regular schedule. Never take two doses at once to "make up" for a missed one, as this can trigger toxic side effects or cardiac issues.
Phenytoin overdose is a medical emergency. Early signs include nystagmus (involuntary eye movement), ataxia (loss of coordination), and dysarthria (slurred speech). Severe overdose can lead to coma, dangerously low blood pressure, and respiratory depression. If an overdose is suspected, contact a poison control center or emergency services immediately.
> Important: Follow your healthcare provider's dosing instructions exactly. Do not adjust your dose or stop taking this medication without medical guidance, as sudden discontinuation can trigger severe, continuous seizures.
Phenytoin is associated with several side effects that occur frequently, especially during the initiation of therapy or when doses are adjusted.
Phenytoin is a potent medication that requires careful management. One of the most critical warnings involves the risk of "Status Epilepticus" if the drug is stopped suddenly. Abrupt discontinuation can cause continuous, life-threatening seizures. Patients must never stop taking Phenytoin or change their dose without the direct supervision of a neurologist. Additionally, Phenytoin has a very narrow therapeutic window, meaning the difference between a dose that works and a dose that is toxic is very small.
Cardiovascular Risk Associated with Rapid Intravenous Administration: The FDA mandates a Black Box Warning for Phenytoin regarding its administration via injection. Rapid intravenous administration of phenytoin is associated with severe cardiovascular events, including atrial and ventricular conduction depression, ventricular fibrillation, and severe hypotension. Deaths have been reported. The rate of administration should not exceed 50 mg per minute in adults. In pediatric patients, the rate should not exceed 1 to 3 mg/kg/min or 50 mg/min, whichever is slower. Close monitoring of cardiac function and blood pressure is essential during and immediately after infusion.
Certain medications should never be used with Phenytoin because the interaction is too dangerous or renders the other drug useless.
Phenytoin is a "potent inducer" of liver enzymes. This means it speeds up the metabolism of other drugs, making them less effective.
Phenytoin must NEVER be used in the following circumstances:
Phenytoin is classified as FDA Pregnancy Category D. This means there is clear evidence of human fetal risk based on adverse reaction data. Use of Phenytoin during pregnancy can cause "Fetal Hydantoin Syndrome," which includes symptoms such as cleft lip and palate, heart defects, and developmental delays. However, having a seizure during pregnancy can also harm the baby by cutting off oxygen. If a woman becomes pregnant while taking Phenytoin, she should not stop the medication abruptly but should contact her neurologist immediately. High-dose folic acid is often recommended for these patients.
Phenytoin is excreted into breast milk in small amounts. Most experts consider it "probably compatible" with breastfeeding, but the infant must be monitored for signs of drowsiness or poor feeding. The decision to breastfeed should be made after a risk-benefit discussion with a pediatrician.
Phenytoin is used in children for the same indications as adults. However, children often have a much faster metabolism, requiring higher mg/kg doses and more frequent administration. Long-term use in children is a concern due to the risk of coarsening facial features and gingival hyperplasia, which can affect a child's self-esteem and dental health.
Phenytoin is a primary membrane stabilizer. Its main molecular target is the voltage-gated sodium channel in the neuronal membrane. Specifically, it binds to the channel in its inactive state, preventing it from reverting to the resting state. This action limits the repetitive firing of action potentials in hyper-excitable neurons. Unlike other drugs, Phenytoin does not interfere with the normal conduction of electrical signals at normal frequencies, which is why it does not cause general anesthesia or profound sedation at therapeutic levels.
The therapeutic effect of Phenytoin is closely related to its concentration in the plasma. The "therapeutic window" is narrow, typically 10 to 20 mcg/mL. Concentrations below this range often fail to prevent seizures, while concentrations above 20 mcg/mL are associated with dose-related toxicity (nystagmus, ataxia, and lethargy). Because the drug's metabolism is saturable, the relationship between dose and plasma concentration is not linear; a 10% increase in dose can lead to a 50% or greater increase in blood levels.
| Parameter | Value |
Common questions about Phenytoin
Phenytoin is a prescription medication primarily used to control and prevent various types of seizures, including generalized tonic-clonic and complex partial seizures. It is also used in hospital settings to treat status epilepticus, which is a medical emergency involving a prolonged seizure state. Additionally, healthcare providers may prescribe it to prevent seizures during or after neurosurgery or following a traumatic brain injury. It works by stabilizing the electrical activity in the brain to prevent the rapid, repetitive firing of neurons that causes a seizure. It is not effective for absence (petit mal) seizures and may actually make them worse.
The most common side effects of Phenytoin involve the central nervous system, such as dizziness, drowsiness, and decreased coordination. Many patients also experience nystagmus, which is an involuntary jerking movement of the eyes, and slurred speech. A very specific and common side effect is gingival hyperplasia, where the gum tissue overgrows the teeth, requiring excellent oral hygiene. Other frequent issues include nausea, vomiting, and constipation. If you notice a skin rash, it is vital to contact your doctor immediately as it could be a sign of a more serious reaction.
Drinking alcohol while taking Phenytoin is generally discouraged because alcohol can unpredictably change the levels of the medication in your blood. Acute or occasional alcohol consumption can slow down the metabolism of Phenytoin, leading to dangerously high levels and toxicity. Conversely, chronic or heavy alcohol use can speed up the metabolism, causing the drug levels to drop and increasing the risk of a breakthrough seizure. Because of these risks, it is safest to avoid alcohol entirely. Always discuss your alcohol consumption habits honestly with your healthcare provider.
Phenytoin is associated with significant risks during pregnancy and is classified as Category D, meaning there is evidence of risk to the human fetus. It can cause a group of birth defects known as Fetal Hydantoin Syndrome, which may include heart defects, cleft lip, and growth problems. However, uncontrolled seizures also pose a major risk to both the mother and the baby. If you are planning to become pregnant or find out you are pregnant, do not stop your medication, but contact your neurologist immediately. They will help you weigh the risks and may adjust your dose or switch you to a safer alternative.
The time it takes for Phenytoin to become effective depends on how it is administered. When given intravenously in an emergency, it can begin working within minutes to stop a seizure. For oral therapy, it can take several days to a week of consistent dosing to reach a 'steady state' where the drug level in your blood is high enough to prevent seizures. Doctors often check blood levels about 7 to 10 days after starting the medication or changing a dose. Because it is absorbed slowly, it is not used for the immediate relief of a single, short seizure when taken by mouth.
No, you should never stop taking Phenytoin suddenly unless specifically instructed by your doctor during a medical emergency. Abruptly stopping an anti-epileptic drug can cause 'rebound' seizures or lead to status epilepticus, a life-threatening condition where seizures do not stop. If the medication needs to be discontinued, your healthcare provider will provide a schedule to gradually taper the dose over weeks or months. This allows your brain to adjust and reduces the risk of severe withdrawal seizures. Always ensure you have enough refills so you do not accidentally run out of medication.
If you miss a dose of Phenytoin, take it as soon as you remember. However, if it is almost time for your next scheduled dose, you should skip the missed dose and simply take the next one at the regular time. Do not take two doses at once to make up for the missed one, as this can lead to toxic side effects or heart rhythm problems. Consistency is vital for maintaining stable blood levels to prevent seizures. If you find yourself frequently forgetting doses, talk to your doctor about using a pill organizer or setting an alarm on your phone.
Weight gain is not considered a common or primary side effect of Phenytoin, unlike some other anti-epileptic drugs like valproic acid or gabapentin. Most patients do not experience significant changes in weight while taking this medication. However, Phenytoin can cause other metabolic changes, such as affecting how your body processes Vitamin D and glucose. If you notice unexpected weight changes or swelling (edema), you should discuss this with your healthcare provider. It is important to maintain a healthy diet and lifestyle while managing a seizure disorder.
Phenytoin has a very high number of drug interactions because it is a potent inducer of liver enzymes, which speeds up the removal of other drugs from your body. It can make birth control pills, blood thinners like warfarin, and many other medications less effective. At the same time, other drugs can raise or lower the levels of Phenytoin in your blood, leading to either toxicity or seizures. Because of this, you must tell your doctor and pharmacist about every medication, supplement, and herbal product you take. Regular blood tests are usually required when any new medication is added or removed.
Yes, Phenytoin is available as a generic medication in several forms, including extended-release capsules, chewable tablets, and oral suspension. While generic versions are generally effective, the FDA and many neurologists recommend staying on the same manufacturer's version of Phenytoin once your seizures are controlled. Small differences in how different generic brands are absorbed can cause your blood levels to fluctuate, which might lead to a breakthrough seizure or side effects. If your pharmacy changes your generic brand, notify your doctor so they can decide if your blood levels need to be re-checked.
Other drugs with the same active ingredient (Phenytoin)
> Warning: Stop taking Phenytoin and call your doctor or emergency services immediately if you experience any of the following:
With prolonged use (years), Phenytoin can impact various body systems:
Cardiovascular Risk with Rapid Infusion: The FDA has issued a Black Box Warning for intravenous Phenytoin. If the drug is infused too quickly, it can cause severe hypotension (low blood pressure) and fatal cardiac arrhythmias. The rate of infusion must not exceed 50 mg per minute in adults or 1-3 mg/kg/min in children. Continuous heart monitoring (ECG) is required during IV administration.
Report any unusual symptoms or persistent side effects to your healthcare provider immediately. Regular blood tests are necessary to ensure the drug remains at a safe and effective level.
Because of its complex nature, patients on Phenytoin require regular laboratory testing:
Phenytoin may cause dizziness, coordination problems, and blurred vision. Patients should not drive or operate heavy machinery until they know how the medication affects them. Furthermore, most jurisdictions have specific laws regarding driving for individuals with a history of seizures.
Alcohol has a complex interaction with Phenytoin. Acute alcohol consumption (a single night of drinking) can increase Phenytoin levels, leading to toxicity. Conversely, chronic alcohol use can decrease Phenytoin levels, increasing the risk of breakthrough seizures. It is best to avoid alcohol entirely while taking this medication.
If Phenytoin must be stopped, it should be tapered (gradually reduced) over several weeks or months. Sudden withdrawal is only performed in the event of a severe allergic reaction, and even then, an alternative anticonvulsant is usually started immediately.
> Important: Discuss all your medical conditions, especially heart, liver, or kidney problems, with your healthcare provider before starting Phenytoin.
Phenytoin can interfere with certain medical tests, potentially leading to inaccurate results:
For each major interaction, the mechanism usually involves the induction of Cytochrome P450 enzymes (reducing other drug levels) or competition for albumin binding (increasing active drug levels). The clinical consequence is either a loss of therapeutic effect for the other drug or a sudden spike in Phenytoin toxicity. Management always involves frequent blood level monitoring and dose adjustments by a healthcare professional.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking, as Phenytoin interacts with more drugs than almost any other medication.
In these cases, the doctor must weigh the benefits of seizure control against significant risks:
Patients who are allergic to other anticonvulsants, such as Carbamazepine or Phenobarbital, may also be allergic to Phenytoin. This is known as "aromatic anticonvulsant hypersensitivity syndrome." If a patient has a history of a drug-induced rash with any seizure medication, Phenytoin should be started with extreme caution, if at all.
> Important: Your healthcare provider will evaluate your complete medical history, including your heart health and genetic background, before prescribing Phenytoin.
Elderly patients (65+) are at a significantly higher risk for Phenytoin toxicity. Age-related declines in liver function and lower levels of albumin (the protein that binds the drug) mean that more active drug stays in the system. Geriatric patients are also more susceptible to the balance-related side effects of Phenytoin, which can lead to dangerous falls and hip fractures.
In patients with end-stage renal disease (ESRD) or significant kidney impairment, the drug's ability to bind to protein is reduced. This results in an increased fraction of "free" phenytoin. Standard blood tests may show a "normal" level, but the patient may be experiencing toxicity. Doctors must monitor "free phenytoin levels" specifically in this population.
Since the liver is the primary site of Phenytoin metabolism, any degree of liver failure (Child-Pugh Class A, B, or C) requires a dose reduction. In severe liver disease, the half-life of the drug can extend from 24 hours to over 100 hours, leading to rapid accumulation.
> Important: Special populations require individualized medical assessment and more frequent blood monitoring.
| Bioavailability | 90% to 95% (Oral) |
| Protein Binding | 90% (primarily to Albumin) |
| Half-life | 7 to 42 hours (average 24h; dose-dependent) |
| Tmax | 1.5 to 12 hours (formulation dependent) |
| Metabolism | Hepatic (CYP2C9 and CYP2C19) |
| Excretion | Renal (<5% unchanged), Fecal (minimal) |
Phenytoin is the prototypical member of the Hydantoin class of anti-epileptic drugs. Related medications include Fosphenytoin (an injectable prodrug) and Ethotoin. It is distinct from other classes like Carbamazepine (iminostilbenes) or Valproate (carboxylic acids).