Tonic-clonic seizures: Mainly used in the prophylactic management of tonic-clonic seizures with complex symptomatology (psychomotor seizures). A period of 5–10 days of dosing may be required to achieve anticonvulsant effects.[citation needed]
Focal seizures: Mainly used to protect against the development of focal seizures with complex symptomatology (psychomotor andtemporal lobe seizures). Also effective in controlling focal seizures with autonomic symptoms.[citation needed]
Absence seizures: Not used in treatment of pure absence seizures due to risk for increasing frequency of seizures. However, can be used in combination with other anticonvulsants during combined absence and tonic-clonic seizures.[citation needed]
Seizures during surgery: A 2018 meta-analysis found that early antiepileptic treatment with either phenytoin or phenobarbital reduced the risk of seizure in the first week after neurosurgery for brain tumors.[15]
Though phenytoin has been used to treat seizures in infants, as of 2023, its effectiveness in this age group has been evaluated in only one study. Due to the lack of a comparison group, the evidence is inconclusive.[17]
Phenytoin has a narrowtherapeutic index. Its therapeutic range for both anticonvulsant and antiarrhythmic effect is 10–20 μg/mL.
Avoid giving intramuscular formulation unless necessary due to skin cell death and local tissue destruction.
Elderly patients may show earlier signs of toxicity.
In the obese, ideal body weight should be used for dosing calculations.
Pregnancy:Pregnancy category D due to risk offetal hydantoin syndrome and fetal bleeding. However, optimal seizure control is very important during pregnancy so drug may be continued if benefits outweigh the risks. Due to decreased drug concentrations as a result ofplasma volume expansion during pregnancy, dose of phenytoin may need to be increased if only option for seizure control.
Breastfeeding: The manufacturer does not recommend breastfeeding since low concentrations of phenytoin are excreted in breast milk.[20]
Liver disease: Do not use oral loading dose. Consider using decreased maintenance dose.
Kidney disease: Do not use oral loading dose. Can begin with standard maintenance dose and adjust as needed.
Severelow blood pressure and abnormal heart rhythms can be seen with rapid infusion of IV phenytoin. IV infusion should not exceed 50 mg/min in adults or 1–3 mg/kg/min (or 50 mg/min, whichever is slower) in children. Heart monitoring should occur during and after IV infusion. Due to these risks, oral phenytoin should be used if possible.[21]
At therapeutic doses, phenytoin may producenystagmus on lateral gaze. At toxic doses, patients experience vertical nystagmus,double vision,sedation, slurred speech, cerebellarataxia, andtremor.[22] If phenytoin is stopped abruptly, this may result in increased seizure frequency, includingstatus epilepticus.[21][20]
Phenytoin may accumulate in thecerebral cortex over long periods of time which can causeatrophy of thecerebellum. The degree of atrophy is related to the duration of phenytoin treatment and is not related to dosage of the medication.[23]
Phenytoin is a knownteratogen, since children exposed to phenytoin are at a higher risk ofbirth defects than children born to women without epilepsy and to women with untreated epilepsy.[30][31] The birth defects, which occur in approximately 6% of exposed children, includeneural tube defects,heart defects andcraniofacial abnormalities, including broad nasal bridge, cleft lip and palate, andsmaller than normal head.[31][32] The effect on IQ cannot be determined as no study involves phenytoin as monotherapy, however poorer language abilities anddelayed motor development may have been associated with maternal use of phenytoin during pregnancy.[30] This syndrome resembles the well-describedfetal alcohol syndrome.[33] and has been referred to as "fetal hydantoin syndrome". Some recommend avoiding polytherapy and maintaining the minimal dose possible during pregnancy, but acknowledge that current data fails to demonstrate a dose effect on the risk of birth defects.[30][31] Data now being collected by the Epilepsy and Antiepileptic Drug Pregnancy Registry may one day answer this question definitively.
There is no good evidence to suggest that phenytoin is a humancarcinogen.[34][35] However, lymph node abnormalities have been observed, including malignancies.[36]
Phenytoin has been associated with drug-inducedgingival enlargement (overgrowth of the gums), probably due to above-mentioned folate deficiency; indeed, evidence from arandomized controlled trial suggests that folic acid supplementation can prevent gingival enlargement in children who take phenytoin.[37] Plasma concentrations needed to induce gingival lesions have not been clearly defined. Effects consist of the following: bleeding upon probing, increased gingivalexudate, pronounced gingival inflammatory response to plaque levels, associated in some instances with bone loss but without tooth detachment.
Phenytoin therapy has been linked to the life-threatening skin reactionsStevens–Johnson syndrome (SJS) andtoxic epidermal necrolysis (TEN). These conditions are significantly more common in patients with a particularHLA-Ballele,HLA-B*1502.[38] This allele occurs almost exclusively in patients with ancestry across broad areas of Asia, including South Asian Indians.
Phenytoin is primarily metabolized to its inactive form by the enzymeCYP2C9. Variations within the CYP2C9 gene that result in decreased enzymatic activity have been associated with increased phenytoin concentrations, as well as reports of drug toxicities due to these increased concentrations.[39] TheU.S. Food and Drug Administration (FDA) notes on the phenytoin drug label that since strong evidence exists linking HLA-B*1502 with the risk of developing SJS or TEN in patients takingcarbamazepine, consideration should be given to avoiding phenytoin as an alternative to carbamazepine in patients carrying this allele.[40]
Phenytoin may increase risk of suicidal thoughts or behavior. People on phenytoin should be monitored for any changes in mood, the development or worsening depression, and/or any thoughts or behavior of suicide.[20]
Chronic phenytoin use has been associated with decreased bone density and increased bone fractures. Phenytoin induces metabolizing enzymes in the liver. This leads to increased metabolism ofvitamin D, thus decreased vitamin D levels.Vitamin D deficiency, as well aslow calcium andphosphate in the blood cause decreased bone mineral density.[20]
Phenytoin is an inducer of theCYP3A4 andCYP2C9 families of theP450 enzyme responsible for the liver's degradation of various drugs.[43]
A 1981 study by theNational Institutes of Health showed thatantacids administered concomitantly with phenytoin "altered not only the extent of absorption but also appeared to alter the rate of absorption. Antacids administered in a peptic ulcer regimen may decrease the AUC of a single dose of phenytoin. Patients should be cautioned against concomitant use of antacids and phenytoin."[44]
Warfarin andtrimethoprim increase serum phenytoin levels and prolong the serum half-life of phenytoin by inhibiting its metabolism. Consider using other options if possible.[45]
In general, phenytoin can interact with the following drugs:[citation needed]
The mechanism of action of phenytoin sodium. Sodium channels are: 1) Closed 2) Open 3) Inactive (phenytoin effect)
Phenytoin is believed to protect against seizures by causing voltage-dependent block ofvoltage gated sodium channels.[46] This blocks sustained high frequency repetitive firing ofaction potentials. This is accomplished by reducing the amplitude of sodium-dependent action potentials through enhancing steady-state inactivation. Sodium channels exist in three main conformations: the resting state, the open state, and the inactive state.
Phenytoin binds preferentially to the inactive form of the sodium channel. Because it takes time for the bound drug to dissassociate from the inactive channel, there is a time-dependent block of the channel. Since the fraction of inactive channels is increased by membranedepolarization as well as by repetitive firing, the binding to the inactive state by phenytoin sodium can produce voltage-dependent, use-dependent and time-dependent block of sodium-dependent action potentials.[47]
The primary site of action appears to be themotor cortex where spread of seizure activity is inhibited.[48] Possibly by promoting sodium efflux from neurons, phenytoin tends to stabilize the threshold against hyperexcitability caused by excessive stimulation or environmental changes capable of reducing membrane sodium gradient. This includes the reduction ofpost-tetanic potentiation at synapses which prevents cortical seizure foci from detonating adjacent cortical areas. Phenytoin reduces the maximal activity of brain stem centers responsible for the tonic phase of generalized tonic-clonic seizures.[21]
Phenytoin elimination kinetics show mixed-order, non-linear elimination behaviour at therapeutic concentrations. Where phenytoin is at low concentration it is cleared byfirst order kinetics, and at high concentrations byzero order kinetics. A small increase in dose may lead to a large increase in drug concentration as elimination becomes saturated. The time to reach steady state is often longer than 2 weeks.[49][50][51][52]
Phenytoin (diphenylhydantoin) was first synthesized by German chemistHeinrich Biltz in 1908.[53]Biltz sold his discovery toParke-Davis, which did not find an immediate use for it. In 1938, other physicians, includingH. Houston Merritt andTracy Putnam, discovered phenytoin's usefulness for controlling seizures, without the sedative effects associated withphenobarbital.[54]
According toGoodman and Gilman's Pharmacological Basis of Therapeutics:
In contrast to the earlier accidental discovery of the antiseizure properties ofpotassium bromide and phenobarbital, phenytoin was the product of a search among nonsedative structural relatives of phenobarbital for agents capable of suppressing electroshock convulsions in laboratory animals.[55]
It was approved by the FDA in 1953 for use in seizures.[citation needed]
Jack Dreyfus, founder of theDreyfus Fund, became a major proponent of phenytoin as a means to control nervousness anddepression when he received a prescription for Dilantin in 1966. He has claimed to have supplied large amounts of the drug toRichard Nixon throughout the late 1960s and early 1970s, although this is disputed by former White House aides[56] and Presidential historians.[57]Dreyfus' experience with phenytoin is outlined in his book,A Remarkable Medicine Has Been Overlooked.[58] Despite more than $70 million in personal financing, his push to see phenytoin evaluated for alternative uses has had little lasting effect on the medical community. This was partially becauseParke-Davis was reluctant to invest in a drug nearing the end of its patent life, and partially due to mixed results from various studies.[citation needed]
In 2008, the drug was put on the FDA's Potential Signals of Serious Risks List to be further evaluated for approval. The list identifies medications with which the FDA has identified potential safety issues, but has not yet identified a causal relationship between the drug and the listed risk. To address this concern, the Warnings and Precautions section of the labeling for Dilantin injection was updated to include additional information aboutPurple glove syndrome in November 2011.[59]
Phenytoin is available as a generic medication.[12]
Since September 2012, the marketing licence in the UK has been held by Flynn Pharma Ltd, of Dublin,Ireland, and the product, although identical, has been called Phenytoin Sodiumxxmg Flynn Hard Capsules. (Thexxmg in the name refers to the strength—for example "Phenytoin sodium 25 mg Flynn Hard Capsules").[60] The capsules are still made byPfizer's Goedecke subsidiary's plant inFreiburg, Germany, and they still have Epanutin printed on them.[61] After Pfizer's sale of the UK marketing licence to Flynn Pharma, the price of a 28-pack of 25 mg phenytoin sodium capsules marked Epanutin rose from 66p (about $0.88) to£15.74 (about $25.06). Capsules of other strengths also went up in price by the same factor—2,384%,[62] costing the UK'sNational Health Service an extra£43 million (about $68.44 million) a year.[63] The companies were referred to theCompetition and Markets Authority (CMA) who found that they had exploited their dominant position in the market to charge "excessive and unfair" prices.[64]
The CMA imposed a record£84.2 million fine on the manufacturer Pfizer, and a£5.2 million fine on the distributor Flynn Pharma and ordered the companies to reduce their prices.[65]
Tentative evidence suggests that topical phenytoin is useful in wound healing in people with chronic skin wounds.[69][70] A meta-analysis also supported the use of phenytoin in managing various ulcers.[71] Phenytoin is incorporated into compounded medications to optimize wound treatment, often in combination withmisoprostol.[72][73]
^World Health Organization (2019).World Health Organization model list of essential medicines: 21st list 2019. Geneva: World Health Organization.hdl:10665/325771. WHO/MVP/EMP/IAU/2019.06. License: CC BY-NC-SA 3.0 IGO.
^"Phenytoin".Lexi-Comp Online.Archived from the original on 4 March 2016. Retrieved18 April 2014.
^Treadwell JR, Wu M, Tsou AY (2022). "Management of Infantile Epilepsies".AHRQ Comparative Effectiveness Reviews. Rockville (MD): Agency for Healthcare Research and Quality (US).doi:10.23970/ahrqepccer252.PMID36383706.S2CID254357105. Report No.: 22(23)-EHC004 2021-SR-01.
^Beckmann CR, et al. (2002).Obstetrics and Gynecology (4th ed.). Baltimore: Lippincott Williams & Wilkins.
^National Task Force on Fetal Alcohol Syndrome and Fetal Alcohol Effect (July 2004)."Fetal Alcohol Syndrome: Guidelines for Referral and Diagnosis"(PDF).National Center on Birth Defects and Developmental Disabilities. Centers for Disease Control and Prevention; U.S. Department of Health and Human Services.
^Report on Carcinogens, Eleventh Edition (PB2005-104914, 2004) p III-216.
^Maeda T, Sano N, Togei K, Shibata M, Izumi K, Otsuka H (1988). "Lack of carcinogenicity of phenytoin in (C57BL/6 x C3H)F1 mice".Journal of Toxicology and Environmental Health.24 (1):111–119.doi:10.1080/15287398809531144.PMID3373561.
^Cuttle L, Munns AJ, Hogg NA, Scott JR, Hooper WD, Dickinson RG, et al. (August 2000). "Phenytoin metabolism by human cytochrome P450: involvement of P450 3A and 2C forms in secondary metabolism and drug-protein adduct formation".Drug Metabolism and Disposition.28 (8):945–950.doi:10.1016/S0090-9556(24)15168-9.PMID10901705.
^Patsalos PN (2012). "Chapter 67 Antiepileptic drug pharmacokinetics and therapeutic drug monitoring".Antiepileptic drug pharmacokinetics and therapeutic drug monitoring. Cambridge University Press. pp. 358–366.
^"Brands".Viatris. 16 November 2020. Retrieved17 June 2024.
^Shaw J, Hughes CM, Lagan KM, Bell PM (November 2007). "The clinical effect of topical phenytoin on wound healing: a systematic review".The British Journal of Dermatology.157 (5):997–1004.doi:10.1111/j.1365-2133.2007.08160.x.PMID17854378.S2CID23862219.
^Bhatia A, Prakash S (July 2004). "Topical phenytoin for wound healing".Dermatology Online Journal.10 (1): 5.doi:10.5070/D30Z3612W1.PMID15347487.
^Riepl M (2022). "A Compendium of Compounding Agents and Formulations, Part 6: Additional Preparations for Refractory Dermal-wound Healing".International Journal of Pharmaceutical Compounding.26 (6):480–488.PMID36445767.
