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Fosphenytoin Side Effects

In Summary

Commonly reported side effects of fosphenytoin include: pruritus, dizziness, drowsiness, ataxia, nystagmus, burning sensation at injection site, and pain at injection site. Other side effects include: stupor, hypotension, paresthesia, tongue disease, pelvic pain, headache, tinnitus, nausea, extrapyramidal reaction, vasodilatation, xerostomia, and ecchymoses. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to fosphenytoin: injection solution

In addition to its needed effects, some unwanted effects may be caused by fosphenytoin. In the event that any of these side effects do occur, they may require medical attention.

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

More common:
  • Blurred vision
  • confusion
  • dizziness, faintness, or lightheadedness when getting up suddenly from a lying or sitting position
  • feeling of warmth or heat
  • flushing or redness of the skin, especially on the face and neck
  • headache
  • sweating
  • unusual tiredness or weakness
Less common:
  • Anxiety
  • bruising
  • burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
  • changes in patterns and rhythms of speech
  • convulsions
  • difficulty with speaking
  • drooling
  • dry mouth
  • fast, pounding, or irregular heartbeat or pulse
  • hyperventilation
  • irritability
  • itching skin
  • lack of coordination
  • large, flat, blue, or purplish patches in the skin
  • loss of balance control
  • muscle trembling, jerking, or stiffness
  • nervousness
  • problems with movement, walking, or speech
  • restlessness
  • shakiness in the legs, arms, hands, or feet
  • shortness of breath
  • shuffling walk
  • slurred speech
  • stiffness of the limbs
  • trouble with sleeping
  • twisting movements of the body
  • uncontrolled movements, especially of the face, neck, and back
  • unsteadiness, trembling, or other problems with muscle control or coordination
  • vomiting
  • Abdominal or stomach pain
  • absence of or decrease in body movement
  • aggressive or angry
  • agitation
  • black, tarry stools
  • bleeding gums
  • bleeding, blistering, burning, coldness, discoloration of the skin, feeling of pressure, hives, infection, inflammation, itching, lumps, numbness, pain, rash, redness, scarring, soreness, stinging, swelling, tenderness, tingling, ulceration, or warmth at the injection site
  • blistering, burning, crusting, dryness, or flaking of the skin
  • blood in the urine or stools
  • bloody nose
  • bluish color of the fingernails, lips, skin, palms, or nail beds
  • body aches or pain
  • bone pain
  • burning while urinating
  • change in consciousness
  • change in personality
  • change in the ability to see colors, especially blue or yellow
  • change in the color of the treated skin
  • chest pain or discomfort
  • cloudy urine
  • cold, clammy skin
  • congestion
  • cough or hoarseness
  • cough producing mucus
  • coughing or spitting up blood
  • crying
  • decrease in frequency of urination
  • decrease in the amount of urine
  • deep or fast breathing with dizziness
  • delusions
  • dementia
  • depression
  • diarrhea
  • difficulty in passing urine (dribbling)
  • difficulty with breathing
  • difficulty with moving
  • difficulty with swallowing
  • dilated neck veins
  • dizziness, fainting, or lightheadedness
  • drowsiness
  • dryness or soreness of the throat
  • dysphoria
  • euphoria
  • eye pain
  • feeling of unreality
  • feeling that others are watching you or controlling your behavior
  • feeling that others can hear your thoughts
  • feeling, seeing, or hearing things that are not there
  • fever or chills
  • flushed, dry skin
  • frequent urination
  • fruit-like breath odor
  • general feeling of discomfort or illness
  • general physical wasting or malnutrition associated with severe illness
  • greatly increased frequency of urination or amount of urine
  • headache, severe and throbbing
  • hives or welts
  • inability to move the legs or arms
  • inability to sit still
  • increase in body movements
  • increase in heart rate
  • increased hunger
  • increased sensitivity of the skin to sunlight
  • increased thirst
  • increased urination
  • increased volume of pale, dilute urine
  • increased watering of the mouth
  • irregular breathing
  • itching, scaling, severe redness, soreness, or swelling of the skin
  • joint pain
  • leg cramps
  • loss of appetite
  • loss of bladder control
  • loss of consciousness
  • loss of memory
  • loss of strength or energy
  • lower back or side pain
  • muscle aches, pains, or stiffness
  • muscle twitching or jerking
  • muscular tenderness, wasting, or weakness
  • nausea or vomiting
  • need to keep moving
  • no blood pressure or pulse
  • not breathing
  • numbness of the feet, hands, and around the mouth
  • numbness or tingling in the face, arms, hands, feet, or lips
  • pain or tenderness around the eyes and cheekbones
  • pale skin
  • paralysis of one side of the body
  • paranoia
  • pinpoint red spots on the skin
  • pounding in the ears
  • problems with memory
  • quick to react or overreact emotionally
  • rapid breathing
  • rapid heartbeat
  • rapid weight gain
  • rapidly changing moods
  • rash with flat lesions or small raised lesions on the skin
  • redness of the skin
  • redness or other discoloration of the skin
  • rhythmic movement of the muscles
  • seizures
  • sense of detachment from self or body
  • severe constipation
  • severe headache
  • severe mood or mental changes
  • severe pain in the chest
  • severe sunburn
  • severe vomiting
  • shivering
  • sleeplessness
  • small lumps under the skin
  • small red or purple spots on the skin
  • sneezing
  • sores, ulcers, or white spots on the lips or in the mouth
  • spots on your skin resembling a blister or pimple
  • stiff neck
  • stopping of heart
  • stuffy or runny nose
  • sudden onset of severe breathing difficulty
  • sudden shortness of breath or troubled breathing
  • sunken eyes
  • swelling of the face, fingers, feet, ankles, or lower legs
  • swelling of the tongue
  • swollen joints
  • swollen, painful, or tender lymph glands in the neck, armpit, or groin
  • tightness of the chest or wheezing
  • troubled breathing with exertion
  • twisting movements of body
  • unable to sleep
  • unconsciousness
  • uncontrolled movements, especially of the face, neck, and back
  • unexplained weight loss
  • unusual behavior
  • unusual bleeding or bruising
  • unusual weak feeling
  • voice changes
  • vomiting of blood or material that looks like coffee grounds
  • weakness or heaviness of the legs
  • weight gain
  • wrinkled skin

Minor Side Effects

Some of the side effects that can occur with fosphenytoin may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

More common:
  • Continuing ringing or buzzing or other unexplained noise in the ears
  • decreased awareness or responsiveness
  • hearing loss
  • severe sleepiness
  • sleepiness or unusual drowsiness
  • uncontrolled eye movements
Less common:
  • Back pain
  • change in taste or bad, unusual, or unpleasant (after) taste
  • change in vision
  • double vision
  • feeling of constant movement of self or surroundings
  • impaired vision
  • lack or loss of strength
  • pelvic pain
  • seeing double
  • sensation of spinning
  • Acid or sour stomach
  • belching
  • bigger, dilated, or enlarged pupils (black part of eye)
  • bloated
  • burning feeling in the chest or stomach
  • burning, dry, or itching eyes
  • change in color vision
  • difficulty having a bowel movement (stool)
  • difficulty seeing at night
  • discharge, excessive tearing
  • ear pain
  • excess air or gas in the stomach or intestines
  • frequent urge to defecate
  • full feeling
  • heartburn
  • increased sense of hearing
  • increased sensitivity of the eyes to light
  • indigestion
  • itching of the vagina or genital area
  • loss of taste
  • pain during sexual intercourse
  • passing gas
  • redness, pain, swelling of the eye, eyelid, or inner lining of the eyelid
  • stomach discomfort, upset, or pain
  • straining while passing stool
  • tenderness in the stomach area
  • thick, white vaginal discharge with no odor or with a mild odor
  • transient, mild, or pleasant aromatic odor
  • vaginal yeast infection
  • weight loss

For Healthcare Professionals

Applies to fosphenytoin: injectable solution


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]


Common (1% to 10%): Hypotension, vasodilation, tachycardia
Uncommon (0.1% to 1%): Shock, cardiac arrest, syncope, cerebral haemorrhage, 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[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]


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%): 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[Ref]


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


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


Frequency not reported: Hypersensitivity syndrome, periarteritis nodosa, SJS/TEN risk in patients of Asian ancestry who have HLA-B 1502
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%): 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, taste loss,reflexes decreased, reflexes increased, migraine, speech disorder, 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%): Asthenia, deafness, accidental injury, fever, face edema, hyperacusis, parosmia, ear pain
Uncommon (0.1% to 1%): Malaise[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%): Agitation, thinking abnormal, nervousness, depression
Uncommon (0.1% to 1%): Confusion, insomnia, depersonalization, psychosis, emotional lability, personality disorder, hostility, amnesia, neurosis, euphoric mood[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]


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


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

2. Keegan MT, Bondy LR, Blackshear JL, Lanier WL "Hypocalcemia-like electrocardiographic changes after administration of intravenous fosphenytoin." Mayo Clin Proc 77 (2002): 584-6

Not all side effects for fosphenytoin may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

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.