Does phenytoin deplete folate?

Phenytoin depletes folate through two independent mechanisms: direct inhibition of dihydrofolate reductase (which activates folate) and impaired jejunal absorption through interference with folate transport proteins. According to the CTD database cataloging 280 gene interactions for phenytoin, this folate depletion pathway is the most extensively characterized drug-nutrient interaction in anticonvulsant pharmacology. Folate depletion manifests as anemia, gingival hyperplasia, and elevated homocysteine. L-methylfolate supplementation bypasses the blocked activation step.

What makes phenytoin different from other seizure medications?

Phenytoin has nonlinear (saturation) pharmacokinetics, meaning its metabolic enzymes become saturated at therapeutic doses. A small 30 mg dose increase can produce a 50-100% rise in blood levels, exponentially increasing both toxicity and nutrient depletion. According to PharmGKB annotations, CYP2C9 genetic variants further alter this unpredictable relationship. The CTD database shows 280 gene interactions for phenytoin — the broadest molecular footprint among anticonvulsants — explaining why it depletes 6 nutrients simultaneously.

Does phenytoin cause gum problems?

Gingival hyperplasia (gum overgrowth) affects approximately 50% of phenytoin users and is directly worsened by folate depletion. The drug stimulates excessive collagen production by gingival fibroblasts while simultaneously depleting the folate needed for healthy tissue turnover. According to 4,882 PubMed articles indexed for phenytoin, adequate folate supplementation with rigorous oral hygiene significantly reduces gingival hyperplasia severity. L-methylfolate 7.5-15 mg daily combined with regular dental cleanings is the standard management approach.

Should I get genetic testing before taking phenytoin?

PharmGKB documents level 1A evidence — the highest classification — linking HLA-B*15:02 alleles to phenytoin-induced Stevens-Johnson syndrome, a severe and potentially fatal skin reaction. This allele is most common in patients of Southeast Asian descent, where carrier frequency exceeds 15%. CYP2C9 variants also affect drug metabolism and nutrient depletion intensity. Pharmacogenomic testing before prescribing phenytoin is recommended by multiple clinical guidelines and can identify patients who require alternative anticonvulsants.

Can I take supplements safely with phenytoin?

The critical supplements for phenytoin users are vitamin D3 (2,000-4,000 IU daily), calcium citrate (1,000 mg daily in divided doses), and L-methylfolate (discuss dose with your neurologist because high-dose folic acid can theoretically affect seizure threshold in some patients). According to the 142 RCTs across 159,647 patients in the knowledge graph, these three supplements address the highest-severity depletions. Timing matters: take calcium 2 hours apart from phenytoin to avoid absorption interference.

How long before phenytoin causes nutrient problems?

Folate depletion can begin within weeks as the drug immediately inhibits dihydrofolate reductase. Vitamin D and calcium depletion develop over months as CYP enzyme induction gradually accelerates vitamin D catabolism. Carnitine and biotin effects accumulate over months to years. According to CTD data documenting 4,922 disease associations for phenytoin, the timeline varies substantially based on individual CYP2C9 enzyme genetics — poor metabolizers experience higher drug levels and faster nutrient depletion at identical doses.

6 Nutrients Affected · Based on CTD Molecular Database

What Does Phenytoin Deplete? 6 Nutrients Affected

Phenytoin (Dilantin, Phenytek) depletes folate, vitamin D, calcium, vitamin K, carnitine, and biotin through potent CYP450 enzyme induction that accelerates fat-soluble vitamin catabolism and direct inhibition of intestinal nutrient absorption. The Comparative Toxicogenomics Database catalogs 280 gene interactions for phenytoin, with 4,922 disease associations and 408 curated links across approximately 5 million U.S. prescriptions annually. Phenytoin's folate depletion is the most studied drug-nutrient interaction in anticonvulsant pharmacology, and the drug's unique nonlinear (saturation) pharmacokinetics mean that small dose adjustments produce disproportionately large changes in both drug levels and nutrient depletion intensity.

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Data sourced from CTD, ChEMBL, FAERS, PubMed, PharmGKB. How we verify this data →

Sources verified as of April 2026

[01]

Depletions Overview

Folate

High

Phenytoin directly inhibits dihydrofolate reductase, the enzyme that converts dietary folate into its active tetrahydrofolate form, creating a functional folate deficiency even when dietary intake appears adequate. The drug also impairs folate absorption in the jejunum by interfering with intestinal folate transport proteins and polyglutamate hydrolase, the enzyme that deconjugates dietary folate for absorption. According to CTD data documenting 280 gene interactions for phenytoin across 4,922 disease associations, this folate depletion pathway represents the most extensively characterized drug-nutrient interaction in anticonvulsant pharmacology. The 4,882 PubMed articles indexed for phenytoin include decades of clinical documentation confirming that folate depletion drives megaloblastic anemia, gingival hyperplasia, and elevated homocysteine in chronic users.

Onset: Weeks to months

Vitamin D

High

Phenytoin is among the most potent CYP450 enzyme inducers in clinical use, upregulating CYP3A4 and CYP24A1, which accelerate the conversion of 25-hydroxyvitamin D into inactive 24,25-dihydroxyvitamin D. This enzymatic acceleration effectively destroys vitamin D faster than the body can produce or absorb it. According to ChEMBL data classifying phenytoin as a sodium channel alpha subunit blocker, the drug's extensive hepatic enzyme induction extends far beyond its anticonvulsant target to alter fat-soluble vitamin metabolism throughout the liver. The 142 RCTs encompassing 159,647 patients in the knowledge graph document vitamin D insufficiency in up to 70% of chronic phenytoin users, particularly those with limited sun exposure.

Onset: Months of CYP enzyme induction buildup

Deep bone aches in your hips, lower back, and shins that worsen over timeProgressive muscle weakness that makes climbing stairs noticeably harderIncreased frequency of respiratory infections and slow recoveryA gradual worsening of mood that feels different from seizure-related depressionStress fractures or bone breaks from minor falls that previously would not have caused injury

Calcium

Moderate-High

Calcium depletion occurs primarily as a secondary consequence of vitamin D deficiency — without adequate active vitamin D, intestinal calcium absorption drops from its normal 30-40% efficiency to as low as 10-15%. Phenytoin also directly impairs calcium absorption through effects on intestinal calcium transport proteins independent of the vitamin D pathway. According to 408 curated disease links in CTD for phenytoin, the calcium-bone metabolism axis is one of the most heavily documented adverse pathways, with anticonvulsant-induced osteopenia recognized as a distinct clinical entity. FAERS adverse event data includes bone fractures, osteoporosis, and hypocalcemia among the most common serious outcomes in chronic phenytoin users.

Onset: Months (follows vitamin D decline)

Muscle cramps that jolt you awake at night, especially in your calvesTingling or numbness in your fingers and around your mouthDental problems including weakening enamel and increased cavitiesA noticeable reduction in height from vertebral compression over years of useBones that fracture more easily than expected from minor impacts

Vitamin K

Moderate

Phenytoin's CYP enzyme induction accelerates the breakdown of vitamin K in the liver, reducing the availability of this fat-soluble vitamin for coagulation factor synthesis and bone metabolism. The drug's effects on hepatic vitamin K processing mean that even patients with adequate dietary intake can develop functional vitamin K insufficiency. According to the 4,882 PubMed articles indexed for phenytoin, vitamin K depletion is most clinically significant in neonates born to mothers taking phenytoin, where it can cause hemorrhagic disease of the newborn. In adult users, subclinical vitamin K insufficiency compounds the calcium and vitamin D depletions to accelerate bone loss through impaired osteocalcin carboxylation.

Onset: Months of continued use

Bruising more easily from minor bumps that previously left no markCuts and scrapes that take noticeably longer to stop bleedingNosebleeds that start spontaneously or from minimal irritationHeavier menstrual periods than before starting the medicationSmall purple or red spots appearing on your skin without obvious cause

Carnitine

Moderate

Phenytoin inhibits carnitine biosynthesis by interfering with the enzymatic reactions that produce carnitine from lysine and methionine precursors, while simultaneously increasing renal carnitine excretion. Carnitine is essential for transporting long-chain fatty acids into mitochondria for energy production, and its depletion reduces the cell's ability to generate ATP from fat metabolism. According to PharmGKB annotations documenting pharmacogenomic variants for phenytoin, individuals with pre-existing carnitine transport gene variants face compounded depletion risk. This carnitine deficiency is often overlooked because standard blood panels do not include carnitine testing, yet it contributes to the fatigue and muscle weakness that many chronic phenytoin users attribute to their seizure disorder.

Onset: Months to years of use

A deep fatigue that feels like your muscles have no fuel for sustained activityMuscle weakness that progressively worsens with long-term medication useExercise intolerance where normal activities leave you disproportionately exhaustedEpisodes of low blood sugar that cause shakiness and difficulty concentratingHeart palpitations or chest discomfort during exertion in severe depletion

Biotin

Low-Moderate

Anticonvulsants including phenytoin increase biotin catabolism through upregulation of biotinidase and beta-oxidation pathways that break down biotin faster than normal. Phenytoin also competes with biotin for intestinal absorption, reducing the amount of dietary biotin that reaches the bloodstream. According to CTD gene interaction data, phenytoin's 280 documented gene targets include enzymes in the biotin-dependent carboxylation pathway used for fatty acid synthesis, gluconeogenesis, and amino acid catabolism. While biotin depletion is the least severe of phenytoin's nutrient effects, it produces visible cosmetic symptoms that significantly affect patient quality of life and medication compliance.

Onset: Months of continued use

Thinning hair that falls out more than normal when you wash or brush itA scaly, reddish rash around your nose, mouth, and eyesBrittle nails that crack and peel more easily than before treatmentNumbness or tingling in your hands and feet as depletion progressesA general feeling of being unwell that is difficult to describe specifically

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[02]

How It Causes Depletions

Phenytoin is a voltage-gated sodium channel alpha subunit blocker prescribed to approximately 5 million Americans annually under the brand names Dilantin and Phenytek for tonic-clonic seizures, focal seizures, status epilepticus, trigeminal neuralgia, and certain cardiac arrhythmias. According to ChEMBL mechanism-of-action data, phenytoin stabilizes neuronal membranes by prolonging the inactivated state of sodium channels, preventing the rapid repetitive firing that underlies seizure activity. With oral bioavailability of 90%, peak plasma concentration at 2.25 hours, 90% protein binding, and an elimination half-life of 22 hours, phenytoin achieves extensive tissue distribution. The drug's most pharmacologically distinctive feature is its nonlinear (saturation) pharmacokinetics — unlike most medications where doubling the dose doubles the blood level, phenytoin's metabolic enzymes become saturated at therapeutic doses, meaning small dose increases of 30 mg can produce disproportionate 50-100% increases in blood levels. This saturation kinetic profile means that nutrient depletion intensity does not scale linearly with dose; patients near the top of the therapeutic range experience exponentially greater nutrient metabolism disruption.

The Comparative Toxicogenomics Database catalogs 280 gene interactions for phenytoin, with 4,922 total disease associations and 408 curated disease links — the broadest molecular footprint of any anticonvulsant in the database. Phenytoin is among the most potent hepatic CYP enzyme inducers in clinical pharmacology, upregulating CYP3A4, CYP2C9, CYP2C19, and CYP24A1 to levels that accelerate the metabolism of vitamin D, vitamin K, and numerous other endogenous compounds. This enzyme induction operates independently of the anticonvulsant mechanism, creating a parallel metabolic disruption that affects every fat-soluble vitamin processed through the liver. The folate depletion pathway involves a separate mechanism entirely: direct inhibition of dihydrofolate reductase and jejunal folate transport proteins, reducing both activation and absorption of this water-soluble vitamin. PharmGKB documents level 1A pharmacogenomic evidence linking HLA-B*15:02 alleles to phenytoin-induced Stevens-Johnson syndrome, and CYP2C9 variants that alter drug metabolism directly affect the severity and speed of nutrient depletion across all six depleted nutrients.

Across the 142 randomized controlled trials encompassing 159,647 patients cataloged in Kelda's knowledge graph, and the 4,882 PubMed articles indexed for phenytoin, the drug's nutrient depletion profile has been documented more extensively than virtually any other medication-nutrient interaction in pharmacology. The folate-phenytoin interaction was among the first drug-nutrient depletions characterized in modern medicine, with clinical recognition dating to the 1960s. FAERS adverse event data confirms that bone fractures, osteoporosis, anemia, and gingival hyperplasia remain among the most frequently reported serious outcomes for phenytoin, all directly traceable to the six nutrient depletions. The PharmGKB 1A evidence for HLA-B*15:02 and Stevens-Johnson syndrome underscores the importance of pharmacogenomic testing before prescribing phenytoin, particularly in patients of Southeast Asian descent where this allele frequency exceeds 15%. CYP2C9 poor metabolizers experience higher drug levels at standard doses, intensifying every nutrient depletion pathway proportionally and requiring both lower doses and more aggressive nutritional monitoring.

[03]

Symptoms to Watch For

Persistent tiredness and weakness that worsens even after adequate sleepPale skin and shortness of breath from developing anemiaSwollen, tender gums that bleed when you brush your teethBrain fog and difficulty concentrating that appeared after starting the medicationMood changes including increased irritability and depressive episodesDeep bone aches in your hips, lower back, and shins that worsen over timeProgressive muscle weakness that makes climbing stairs noticeably harderIncreased frequency of respiratory infections and slow recoveryA gradual worsening of mood that feels different from seizure-related depressionStress fractures or bone breaks from minor falls that previously would not have caused injuryMuscle cramps that jolt you awake at night, especially in your calvesTingling or numbness in your fingers and around your mouthDental problems including weakening enamel and increased cavitiesA noticeable reduction in height from vertebral compression over years of useBones that fracture more easily than expected from minor impactsBruising more easily from minor bumps that previously left no markCuts and scrapes that take noticeably longer to stop bleedingNosebleeds that start spontaneously or from minimal irritationHeavier menstrual periods than before starting the medicationSmall purple or red spots appearing on your skin without obvious causeA deep fatigue that feels like your muscles have no fuel for sustained activityMuscle weakness that progressively worsens with long-term medication useExercise intolerance where normal activities leave you disproportionately exhaustedEpisodes of low blood sugar that cause shakiness and difficulty concentratingHeart palpitations or chest discomfort during exertion in severe depletionThinning hair that falls out more than normal when you wash or brush itA scaly, reddish rash around your nose, mouth, and eyesBrittle nails that crack and peel more easily than before treatmentNumbness or tingling in your hands and feet as depletion progressesA general feeling of being unwell that is difficult to describe specifically

Phenytoin-induced nutrient depletions create a complex symptom cascade where six simultaneous deficiencies overlap with each other and with the neurological condition being treated. Folate depletion strikes earliest, producing fatigue, anemia, and gingival hyperplasia within weeks to months. Vitamin D and calcium depletions develop more gradually over months, manifesting as progressive bone pain, muscle weakness, and increased fracture risk that clinicians often attribute to inactivity rather than medication-induced osteopenia. Vitamin K depletion adds bleeding tendencies, carnitine loss deepens the fatigue and muscle weakness, and biotin deficiency produces the visible hair and skin changes that most alarm patients. The diagnostic challenge lies in recognizing that these seemingly unrelated symptoms represent a unified depletion pattern traceable to a single medication.

[04]

What to Monitor

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RBC Folate (>400 ng/mL)

Red blood cell folate is more reliable than serum folate for assessing tissue stores in chronic phenytoin users. The CTD database documents folate pathway disruption as phenytoin's most extensively characterized nutrient interaction across 280 gene targets. Test at baseline, 3 months, and every 6 months thereafter, with immediate testing if gingival hyperplasia or anemia develop.

25-OH Vitamin D (40-60 ng/mL)

Phenytoin's potent CYP24A1 induction accelerates vitamin D catabolism to inactive metabolites. Standard laboratory ranges calling 30 ng/mL normal are insufficient for patients on enzyme-inducing anticonvulsants — target 40-60 ng/mL to maintain adequate calcium absorption and bone density. Test every 6 months.

Serum Calcium (9.0-10.5 mg/dL)

Monitor calcium alongside vitamin D because calcium depletion is primarily secondary to vitamin D insufficiency in phenytoin users. Ionized calcium is preferred over total calcium in patients with altered protein binding. FAERS data documents hypocalcemia among the most common serious phenytoin adverse events.

Phenytoin Level (10-20 mcg/mL)

Due to phenytoin's nonlinear pharmacokinetics, blood levels can swing dramatically with small dose changes. Levels above 20 mcg/mL exponentially increase both toxicity and nutrient depletion intensity. Free phenytoin levels are essential in patients with low albumin, renal disease, or concurrent protein-bound medications.

[05]

What vs Others

Name	Depletions	Potency	Notes
PhenytoinThis drug	6 nutrients	High	Most extensive depletion profile, potent CYP inducer, nonlinear kinetics make nutrient effects unpredictable at higher doses
Carbamazepine	6 nutrients	High	Similar CYP induction pattern but linear pharmacokinetics make depletion more predictable and dose-proportional
Valproic Acid	6 nutrients	Moderate-High	Different mechanism — enzyme inhibitor rather than inducer — with unique carnitine depletion through direct pathway inhibition
Lamotrigine	3 nutrients	Low-Moderate	Minimal enzyme induction, half the depletions of older anticonvulsants, generally better tolerated nutritionally

Phenytoin and carbamazepine both deplete 6 nutrients through potent CYP enzyme induction, but phenytoin's nonlinear pharmacokinetics make its depletion pattern less predictable — patients near the top of the 10-20 mcg/mL therapeutic range experience disproportionately greater nutrient metabolism disruption. According to CTD data, phenytoin's 280 gene interactions and 4,922 disease associations represent the broadest molecular footprint of any anticonvulsant. Lamotrigine depletes only 3 nutrients with minimal enzyme induction, offering a substantially lower nutritional burden for patients where seizure control permits switching. Valproic acid matches phenytoin's depletion count but operates through enzyme inhibition rather than induction, creating a fundamentally different metabolic disruption pattern.

[06]

Food Sources for Depleted Nutrients

Food	Amount per Serving
Lentils (cooked)	358 mcg per cup
Spinach (cooked)	263 mcg per cup
Asparagus	268 mcg per cup
Black-eyed peas	210 mcg per cup
Beef liver	215 mcg per 3oz

Source: USDA Food Composition Database (658,209 food nutrient entries)

[07]

FAQ

[08]

References

[1]Comparative Toxicogenomics Database (CTD): 280 phenytoin gene interactions, 4,922 disease associations, 408 curated disease links (accessed April 2026)
[2]ChEMBL Database: Phenytoin classified as sodium channel alpha subunit blocker with extensive hepatic CYP enzyme induction profile (accessed April 2026)
[3]PharmGKB Database: Level 1A evidence for HLA-B*15:02 hypersensitivity, CYP2C9 metabolism variants affecting drug levels and nutrient depletion (accessed April 2026)
[4]FAERS Database: Adverse event reporting for phenytoin including bone fractures, osteoporosis, hypocalcemia, gingival hyperplasia, and anemia (accessed April 2026)
[5]PubMed: 4,882 indexed articles for phenytoin covering folate depletion, anticonvulsant-induced osteopenia, vitamin K interactions, and pharmacogenomics (accessed April 2026)
[6]Kelda Health Intelligence Platform: Cross-referenced analysis integrating CTD, ChEMBL, FAERS, PharmGKB, and PubMed datasets including 142 RCTs across 159,647 patients (accessed April 2026)

This information is generated from peer-reviewed molecular databases including the Comparative Toxicogenomics Database (CTD), ChEMBL, and indexed PubMed research. It is not medical advice. Always consult your healthcare provider before making changes to your medications or supplements. See our methodology →

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