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Phenytoin Side Effects

In Summary

Commonly reported side effects of phenytoin include: congenital anomalies. Other side effects include: hepatic necrosis, confusion, fatigue, constipation, vertigo, ataxia, mental status changes, myasthenia, hypertrichosis, nervousness, tremor of hands, numbness, dizziness, drowsiness, tremor, depression, irritability, excitement, mood changes, and restlessness. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to phenytoin: oral capsule, oral capsule extended release, oral suspension, oral tablet chewable

Other dosage forms:

As well as its needed effects, phenytoin may cause unwanted side effects that require medical attention.

Major Side Effects

If any of the following side effects occur while taking phenytoin, check with your doctor immediately:

More common:
  • Decreased coordination
  • mental confusion
  • nervousness
  • slurred speech
  • trouble with breathing, speaking, or swallowing
  • unsteadiness, trembling, or other problems with muscle control or coordination
  • Inability to move the eyes
  • increased blinking or spasms of the eyelid
  • shakiness and unsteady walk shakiness in the legs, arms, hands, or feet
  • sticking out of the tongue
  • trembling or shaking of the hands or feet
  • twitching, twisting, or uncontrolled repetitive movements of the tongue, lips, face, arms, or legs
  • uncontrolled twisting movements of the neck, trunk, arms, or legs
  • unusual facial expressions
Incidence not known:
  • Abdominal or stomach pain
  • bleeding gums
  • blistering, peeling, or loosening of the skin
  • blisters, hives, or itching
  • bloating of the abdomen or stomach
  • blood in the urine or stools
  • bloody, black, or tarry stools
  • chest pain
  • chills
  • cough or hoarseness
  • dark urine
  • diarrhea
  • difficulty with moving
  • fainting spells
  • fever with or without chills
  • general feeling of discomfort or illness
  • general feeling of tiredness or weakness
  • hair loss
  • headache
  • high fever
  • irregular heartbeat
  • irritation in the mouth
  • joint or muscle pain
  • light-colored stools
  • lower back or side pain
  • muscle stiffness
  • nausea or vomiting
  • numbness, tingling, or pain in the hands or feet
  • painful or difficult urination
  • pale skin
  • pinpoint red spots on the skin
  • red skin lesions, often with a purple center
  • red, irritated eyes
  • redness and swelling of the gums
  • skin blisters
  • skin rash
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • swollen, painful, or tender lymph glands in the neck, armpit, or groin
  • unusual bleeding or bruising
  • unusual tiredness
  • upper right abdominal or stomach pain
  • weakness
  • weight loss
  • yellow eyes or skin

Minor Side Effects

Some phenytoin side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

More common:
  • Trouble sleeping
  • uncontrolled eye movements
Incidence not known:
  • Difficulty having a bowel movement (stool)
  • enlarged lips
  • feeling of constant movement of self or surroundings
  • increased hair growth on the forehead, back, arms, or legs
  • pain of the penis on erection
  • sensation of spinning

For Healthcare Professionals

Applies to phenytoin: compounding powder, injectable solution, oral capsule, oral capsule extended release, oral suspension, oral tablet chewable


Common (1% to 10%): Hypotension, vasodilation, tachycardia
Uncommon (0.1% to 1%): Shock, cardiac arrest, syncope, cerebral hemorrhage, palpitation, sinus bradycardia, atrial flutter, bundle branch block, cardiomegaly, cerebral infarct, postural hypotension, pulmonary embolus, QT interval prolongation, thrombophlebitis, ventricular extrasystoles, congestive heart failure
Frequency not reported: Cardiovascular collapse, severe cardiotoxic reactions with atrial and ventricular conduction depression (including bradycardia and all degrees of heart block), asystole ventricular fibrillation, periarteritis nodosa[Ref]


Very common (10% or more): Rash (more serious and rare forms have included bullous, exfoliative or purpuric dermatitis, lupus erythematosus, Stevens-Johnson syndrome and toxic epidermal necrolysis), pruritus
Common (1% to 10%): Maculopapular rash, urticaria, sweating, skin discoloration, contact dermatitis, pustular rash, skin nodule, ecchymosis
Frequency not reported: Hirsutism, hypertrichosis, coarsening of the facial features, enlargement of the lips, Peyronie's disease. Dupuytren's contracture and drug rash with eosinophilia and systemic symptoms (DRESS), erythema multiforme, SJS/TEN risk in patients of Asian ancestry who have HLA-B 1502[Ref]


Frequency not reported: Decreased serum T4 and FT4, increased serum estradiol levels, gynecomastia, alterations in vitamin D metabolism[Ref]


Very common (10% or more): Nausea (13%)
Common (1% to 10%): Tongue disorder, dry mouth, vomiting, constipation
Uncommon (0.1% to 1%): Dyspepsia, diarrhea, anorexia, GI hemorrhage, increased salivation, tenesmus, tongue edema, dysphagia, flatulence, gastritis, ileus
Frequency not reported: Hypoesthesia of the tongue, gingival hyperplasia[Ref]


Uncommon (0.1% to 1%): Urinary retention, oliguria, dysuria, vaginitis, albuminuria, genital edema, polyuria, urethral pain, urinary incontinence, vaginal moniliasis
Frequency not reported: Feeling of warmth or tingling in the groin, Peyronie's disease[Ref]


Common (1% to 10%): Ecchymosis
Uncommon (0.1% to 1%): Thrombocytopenia, anemia, leukocytosis, cyanosis, hypochromic anemia, leukopenia, lymphadenopathy, petechia
Frequency not reported: Granulocytopenia, agranulocytosis, pancytopenia (with or without bone marrow suppression), aplastic anemia[Ref]


Uncommon (0.1% to 1%): Liver function tests abnormal, kidney failure
Frequency not reported: Toxic hepatitis, hepatocellular damage[Ref]


Frequency not reported: Hypersensitivity syndrome, periarteritis nodosa
Postmarketing reports: Anaphylactoid reaction, anaphylaxis[Ref]


Uncommon (0.1% to 1%): Sepsis, flu syndrome, cryptococcosis
Frequency not reported: Immunoglobulin abnormalities[Ref]


Common (1% to 10%): Injection site reaction, injection site pain
Uncommon (0.1% to 1%): Injection site inflammation, injection site edema, injection site hemorrhage[Ref]


Uncommon (0.1% to 1%): Generalized edema, cachexia, diabetes insipidus
Frequency not reported: Hyperglycemia, appetite disorder, hypokalemia[Ref]


Common (1% to 10%): Back pain, chills, myasthenia
Uncommon (0.1% to 1%): Myopathy, leg cramps, arthralgia, myalgia, muscular weakness, muscle twitching, muscle spasms
Frequency not reported: Systemic lupus erythematosus, polyarthritis, Purple Glove Syndrome[Ref]

Nervous system

Very common (10% or more): Nystagmus (59%), dizziness (31%), somnolence (27%), ataxia (18%)
Common (1% to 10%): Headache, stupor, incoordination, paresthesia, extrapyramidal syndrome, tremor, vertigo, brain edema, tinnitus, taste perversion (including metallic taste), taste loss, reflexes decreased, reflexes increased, migraine, slurred speech, dysarthria, intracranial hypertension, hyperesthesia, hyporeflexia
Uncommon (0.1% to 1%): Twitching, Babinski sign positive, circumoral paresthesia, hemiplegia, hypotonia, convulsion, meningitis, CNS depression, hypokinesia, hyperkinesia, brain edema, paralysis, aphasia, coma, myoclonus, acute brain syndrome, encephalitis, subdural hematoma, encephalopathy, akathisia
Frequency not reported: CNS depression, dyskinesia (including chorea), dystonia and asterixis similar to those induced by phenothiazines or other neuroleptic drugs, drowsiness, motor twitching, tonic seizures, sensory peripheral polyneuropathy (in patients receiving long-term therapy)[Ref]


Common (1% to 10%): Diplopia, amblyopia, blurred vision, visual impairment
Uncommon (0.1% to 1%): Photosensitivity reaction, photophobia, eye pain, mydriasis, visual field defect[Ref]


Common (1% to 10%): Asthenia, deafness, accidental injury, fever, face edema, hyperacusis, parosmia, ear pain
Uncommon (0.1% to 1%): Malaise[Ref]


Common (1% to 10%): Pelvic pain
Uncommon (0.1% to 1%): Kidney failure[Ref]


Common (1% to 10%): Pneumonia
Uncommon (0.1% to 1%): Pharyngitis, sinusitis, hyperventilation, rhinitis, apnea, aspiration pneumonia, asthma, dyspnea, atelectasis, cough increased, sputum increased, epistaxis, hypoxia, pneumothorax, hemoptysis, bronchitis
Postmarketing reports: Respiratory arrest (sometimes fatal)[Ref]


Common (1% to 10%): Agitation, thinking abnormal, nervousness, depression
Uncommon (0.1% to 1%): Confusion, insomnia, depersonalization, psychosis, emotional lability, personality disorder, hostility, amnesia, neurosis, euphoric mood[Ref]


1. Cerner Multum, Inc. "UK Summary of Product Characteristics." O 0

2. Pharmaceutical Society of Australia "APPGuide online. Australian prescription products guide online. Available from: URL:" ([2006]):

3. "Product Information. Dilantin (phenytoin)." Parke-Davis, Morris Plains, NJ.

It is possible that some side effects of phenytoin may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.