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Valtrex Side Effects

Generic Name: valacyclovir

Note: This page contains side effects data for the generic drug valacyclovir. It is possible that some of the dosage forms included below may not apply to the brand name Valtrex.

For the Consumer

Applies to valacyclovir: oral tablet

As well as its needed effects, valacyclovir (the active ingredient contained in Valtrex) may cause unwanted side effects that require medical attention.

Major Side Effects

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

More common:
  • Discouragement
  • feeling sad or empty
  • irritability
  • lack of appetite
  • loss of interest or pleasure
  • tiredness
  • trouble concentrating
  • trouble sleeping
  • Black, tarry stools
  • chest pain
  • chills
  • cough
  • decreased frequency or output of urine
  • fever
  • flu-like symptoms
  • headache
  • lower back or side pain
  • reduced mental alertness
  • shortness of breath
  • yellow eyes or skin
Incidence not known:
  • Actions that are out of control
  • agitation
  • anxiety
  • back, leg, or stomach pains
  • bleeding gums
  • blood in urine or stools
  • blurred vision
  • change in consciousness
  • change in mental status
  • changes in behavior, especially in interactions with other people
  • changes in patterns and rhythms of speech
  • dark or bloody urine
  • difficult or labored breathing
  • difficulty speaking
  • difficulty swallowing
  • dizziness
  • drowsiness
  • dry mouth
  • fast, pounding, or irregular heartbeat or pulse
  • 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
  • general tiredness and weakness
  • hyperventilation
  • increased thirst
  • itching
  • lightheadedness when getting up from a lying or sitting position
  • light-colored stools
  • loss of consciousness
  • mood or mental changes
  • nausea and vomiting
  • nervousness
  • pale color of skin
  • pinpoint red spots on the skin
  • pounding in the ears
  • puffiness or swelling of the eyelids or around the eyes, face, lips, or tongue
  • redness of the skin
  • restlessness
  • seeing, hearing, or feeling things that are not there
  • seizures
  • severe mood or mental changes
  • shakiness and unsteady walk
  • shakiness in the legs, arms, hands, or feet
  • skin rash
  • slurred speech
  • sores, ulcers, or white spots on the lips or in the mouth
  • stiff neck
  • swelling of the face, fingers, or lower legs
  • swollen or painful glands
  • talking, feeling, and acting with excitement
  • tightness in the chest
  • trembling or shaking of the hands or feet
  • trouble in speaking
  • troubled breathing
  • unsteadiness, trembling, or other problems with muscle control or coordination
  • unusual behavior
  • unusual bleeding or bruising
  • unusual tiredness or weakness
  • upper right abdominal pain
  • vomiting
  • weight gain
  • wheezing

Minor Side Effects

Some valacyclovir 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:
  • Body aches or pain
  • cramps
  • difficulty in moving
  • ear congestion
  • heavy bleeding
  • loss of voice
  • muscle aches
  • muscle pain or stiffness
  • nasal congestion
  • pain
  • pain in joints
  • sneezing
  • sore throat
  • stuffy or runny nose
Less common:
  • Constipation
  • diarrhea
Incidence not known:
  • Blistering, peeling, or loosening of the skin
  • hair loss or thinning of the hair
  • hives or welts
  • increased sensitivity of skin to sunlight
  • red, irritated eyes
  • redness or other discoloration of the skin
  • severe sunburn

For Healthcare Professionals

Applies to valacyclovir: oral tablet


Gastrointestinal side effects have included nausea (up to 15%), abdominal pain (up to 11%), and vomiting (up to 6%). Constipation, anorexia, diarrhea, elevated amylase, and elevated serum lipase have been reported. Diarrhea has been reported during postmarketing experience. In clinical trials of otherwise healthy individuals, frequencies were higher for patients over 50 years of age than for younger patients.[Ref]

Nervous system

Nervous system side effects have included headache (up to 38%) and dizziness (up to 4%). Central nervous system effects including agitation, seizures, and encephalopathy have been reported. Choreiform movements, myoclonus, vasculitic mononeuritis multiplex, somnolence, and Cotard's syndrome have been reported. Agitation, ataxia, coma, decreased consciousness, dysarthria, encephalopathy, seizures, and tremors have been reported during postmarketing experience. Neurotoxicity has been most commonly reported in patients with renal failure, the elderly, and in patients following bone marrow transplant, and is associated with high serum concentrations of acyclovir.[Ref]

Acyclovir neurotoxicity is almost exclusively seen in patients with renal failure. These patients may have longstanding chronic renal failure, or acute failure which may be attributed to acyclovir. One group of six bone marrow transplant patients exhibited abnormal EEGs with diffuse slowing. Although more commonly seen with intravenous administration of higher doses, neurotoxicity has also been reported in patients receiving oral doses of acyclovir. Following discontinuation of therapy, mental status recovered within about a week. Several patients with chronic renal failure exhibiting neurotoxicity improved dramatically following hemodialysis.

Although there have been no similar reports in clinical trials of valacyclovir completed to date, the assumption may be made that neurotoxicity can also occur with valacyclovir based on the fact that plasma acyclovir concentrations from oral valacyclovir administration tend to be much higher than those obtained with oral acyclovir.[Ref]


Psychiatric side effects have included depression (up to 7%) and disorientation. Central nervous system effects including hallucinations, confusion, and delirium have been reported. Aggressive behavior, confusion, mania, and psychosis (including auditory and visual hallucinations) have been reported during postmarketing experience.


Transient renal dysfunction has been reported with both oral and intravenous administration of acyclovir. Crystallization of the drug in the renal tubules is thought to be the mechanism for the development of renal dysfunction based on findings of crystalluria in several case reports and at least one prospective study. Inadequate hydration of the patient and rapid administration of the drug may contribute to the development of crystalluria. Acute tubular necrosis and interstitial nephritis have also been reported in association with acyclovir therapy. Although there have been no similar reports in clinical trials of valacyclovir (the active ingredient contained in Valtrex) completed to date, the assumption may be made that renal toxicity can also occur with valacyclovir based on the fact that plasma acyclovir concentrations from oral valacyclovir administration tend to be much higher than those obtained with oral acyclovir.[Ref]

Renal side effects have included acute renal failure, elevated serum creatinine (up to 0.7%), renal toxicity, and renal failure (presenting as an increase in serum creatinine and blood urea nitrogen). Renal failure and renal pain (may be associated with renal failure) have been reported during postmarketing experience. Renal effects are transient and resolve over several days following discontinuation of therapy. Renal damage is most likely due to crystallization of acyclovir in the renal tubules. Patients with preexisting renal insufficiency are at greater risk for developing neurotoxicity and further deterioration in renal function.[Ref]


Hematologic side effects have included thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS), and decreased neutrophil counts (18%), platelet counts (up to 3%), hemoglobin (up to 0.8%), and white blood cells (up to 1.3%). Thrombocytopenia, aplastic anemia, leukocytoclastic vasculitis, and TTP/HUS have been reported during postmarketing experience.[Ref]

TTP/HUS, including some fatalities, has been reported during clinical trials in patients with advanced HIV disease and in allogeneic bone marrow transplant and renal transplant recipients, who were receiving 8 g valacyclovir per day.[Ref]


Hepatic side effects have included elevated AST (up to 16%), ALT (up to 14%), and bilirubin. Liver enzyme abnormalities and hepatitis have been reported during postmarketing experience.[Ref]


Hypersensitivity side effects have included acute hypersensitivity reactions (including anaphylaxis, angioedema, dyspnea, pruritus, rash, and urticaria) during postmarketing experience. Stevens-Johnson syndrome has been reported.[Ref]


Dermatologic side effects have included rash (8%). Erythema multiforme, rashes including photosensitivity, and alopecia have been reported during postmarketing experience.[Ref]


Cardiovascular side effects have included hypertension and tachycardia during postmarketing experience.[Ref]


Ocular side effects have included visual abnormalities during postmarketing experience.[Ref]


Respiratory side effects have included nasopharyngitis (16%) and upper respiratory tract infection (9%).[Ref]


Musculoskeletal side effects have included arthralgia (up to 6%).[Ref]


Genitourinary side effects have included dysmenorrhea (up to 8%).[Ref]


Other side effects have included fatigue (8%). Facial edema has been reported during postmarketing experience.[Ref]


Metabolic side effects have included elevated alkaline phosphatase (4%) and hypoglycemia. At least one case of hypercalcemia has been reported.[Ref]


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It is possible that some side effects of Valtrex 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.