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Cerebyx Side Effects

Generic Name: fosphenytoin

Note: This page contains information about the side effects of fosphenytoin. Some of the dosage forms included on this document may not apply to the brand name Cerebyx.

Common side effects of Cerebyx 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 (the active ingredient contained in Cerebyx). 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

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


The severity and/or frequency of adverse side effects due to fosphenytoin (the active ingredient contained in Cerebyx) (and phenytoin, the active metabolite) may be increased in patients with renal and/or hepatic disease or in those with hypoalbuminemia. Fosphenytoin conversion to phenytoin may be increased without a concomitant increase in phenytoin clearance.[Ref]

In general, cardiovascular collapse and central nervous system depression are the most important adverse sequelae and are more common in debilitated patients, during rapid administration rates, and with higher doses (e.g. >15 mg/kg at 150 mg/min).[Ref]

Nervous system

Nervous system side effects commonly reported have included transient paresthesias (described as itching, burning or tingling) at a site distant from the injection site (most commonly the groin area). Somnolence, nystagmus, dizziness, stupor, and ataxia (which may be signs of toxicity) have also been reported.[Ref]

The paresthesia reaction is more common with fosphenytoin than phenytoin and does not appear to be an allergic process. The paresthesias usually begin several minutes within the start of the infusion and resolve within 10 minutes after completion of the infusion. Continuation of the infusion during the paresthesia side effect does not appear to result in permanent adverse effects, although the outcome has not been studied in a controlled fashion.

Other less common nervous system effects have included increased reflexes, speech disorder, dysarthria, intracranial hypertension, and nervousness.[Ref]


Cardiovascular side effects appear to be more common with intravenous (versus intramuscular) administration. Elderly and/or debilitated patients may be at greatest risk for cardiovascular complications.[Ref]

Cardiovascular side effects including hypotension, hypertension, bradycardia, tachycardia, and vasodilation have been reported. A case of hypocalcemia-like ECG changes has also been reported.[Ref]


Gastrointestinal side effects have included nausea, tongue and taste disorder, constipation, dry mouth, vomiting, and gingival hyperplasia.[Ref]


Transient pruritus has been reported to occur more frequently with higher doses and faster infusion rates.

Local injection-site pain tends to be more common with phenytoin than with fosphenytoin (the active ingredient contained in Cerebyx) However, infusion disruptions due to systemic burning, itching and/or paresthesia occur more frequently with fosphenytoin than with phenytoin. Pruritus in the perineum appears to be common. Localized injection site pain and burning result in more infusion discontinuations in patients treated with phenytoin. Intramuscular fosphenytoin is well tolerated, with transient mild to moderate itching being the most common local effect. In general, intravenous infusions of fosphenytoin and phenytoin are completed in 5 to 7 and 15 to 20 minutes, respectively. Shorter infusion times with fosphenytoin may result in less local irritation.[Ref]

Local side effects have included transient pruritus.[Ref]


Tinnitus has been reported with higher doses and infusion rates.[Ref]

Other side effects have included tinnitus, fevers, and infrequent headaches.[Ref]


Serious, sometimes fatal hematologic events have been reported with phenytoin. Thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, lymphadenopathy, and pancytopenia with or without bone marrow suppression have occurred with phenytoin use. Fatalities have been reported. In cases of severe blood dyscrasias, selection of an alternative to fosphenytoin (the active ingredient contained in Cerebyx) therapy may be the most prudent decision.

Lymphadenopathy (local or generalized) has been reported with phenytoin use and while a causal relationship has not been determined, differentiation from other lymph node pathologies should be made. Any patient who develops lymphadenopathy should be observed for an extended period of time and alternative seizure control medications should be instituted if at all possible.[Ref]

Hematologic side effects including ecchymosis have been reported infrequently.[Ref]


Metabolic side effects such as hypokalemia may occur.[Ref]

Slow metabolism (due to a genetic lack of inducing enzyme) has occurred rarely in patients treated with phenytoin.[Ref]


Musculoskeletal side effects have included myasthenia.[Ref]


Dermatologic side effects including pruritus (up to 49% in one major clinical trial), bullous rash, exfoliative dermatitis, gingival hyperplasia, and erythema multiforme have been reported.[Ref]

Pruritus with fosphenytoin is a dose related reaction which has been reported to develop more frequently if the drug is administered intravenously. The groin is the area affected most frequently. The pruritus may last for hours.[Ref]


Hepatic side effects including acute hepatotoxicity have been reported with the use of phenytoin. Therefore, if hepatotoxicity occurs with the use of fosphenytoin (the active ingredient contained in Cerebyx) the drug should be discontinued and not readministered.[Ref]


1. "Product Information. Cerebyx (fosphenytoin)." Parke-Davis, Morris Plains, NJ.

2. Uthman BM, Wilder BJ "Emergency management of seizures: an overview." Epilepsia 2 (1989): 533-7

3. Uthman BM, Wilder BJ, Ramsay RE "Intramuscular use of fosphenytoin: an overview." Neurology 46 (6 supp (1996): s24-8

4. Boucher BA, Feler CA, Dean JC, et al. "The safety, tolerability, and pharmacokinetics of fosphehytoin after intramuscular and intravenous administration in neurosurgery patients." Pharmacotherapy 16 (1996): 638-45

5. Browne TR, Kugler AR, Eldon MA "Pharmacology and pharmacokinetics of fosphenytoin." Neurology 46 (6 supp (1996): s3-7

6. Sloan EP "Emergency department seizure treatment." P&T 21(suppl 5) (1996): s24-9

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

8. Ramsay RE, DeToledo J "Intravenous administration of fosphenytoin: options for the management of seizures." Neurology 46 (6 supp (1996): s17-9

9. Bebin M, Bleck TP "New anticonvulsant drugs: focus on flunarizine, fosphenytoin, midazolam and stiripentol." Drugs 48 (1994): 153-71

10. Hebert AA, Ralston JP "Cutaneous reactions to anticonvulsant medications." J Clin Psychiatry 62 (2001): 22-6

11. Jamerson BD, Dukes GE, Brouwer KL, Donn KH, Messenheimer JA, Powell JR "Venous irritation related to intravenous administration of phenytoin versus fosphenytoin." Pharmacotherapy 14 (1994): 47-52

Not all side effects for Cerebyx 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.