What the label lists and what reports show
Valacyclovir side effects
Valacyclovir's FDA label: “The most common adverse reactions reported in at least 1 indication by greater than 10% of adult subjects treated with VALTREX and observed more frequently with VALTREX compared with placebo are headache, nausea, and abdominal pain.” FAERS holds 70,054 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is fatigue (4,317 reports).
- Generic listed
Check it at the source: the valacyclovir FDA label on DailyMed (effective November 12, 2025, GlaxoSmithKline LLC); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of VALTREX?VALTREX can cause serious side effects including: • Thrombotic Thrombocytopenic Purpura (TTP) and Hemolytic Uremic Syndrome (HUS). TTP and HUS have happened in people with weakened immune systems taking VALTREX and have led to death. TTP and HUS are disorders that can cause small blood clots to form throughout the body and decrease blood flow to body organs such as the brain, heart, and kidneys. Your healthcare provider will stop treatment with VALTREX if you have signs or symptoms of TTP and HUS. • kidney failure. • nervous system problems. Tell your healthcare provider right away if you get any of these signs or symptoms of nervous system problems during treatment with VALTREX: ○aggressive behavior ○unsteady movement ○shaky movements ○confusion ○speech problems ○hallucinations (seeing or hearing things that are really not there) ○seizures ○coma • severe skin reactions. […]
Source: FDA drug label, GlaxoSmithKline LLC, effective November 12, 2025 (SPL set f8e0d8f8-cb73-4206-a484-88f5c4fbd719), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions reported in at least 1 indication by greater than 10% of adult subjects treated with VALTREX and observed more frequently with VALTREX compared with placebo are headache, nausea, and abdominal pain.
Table 2. Incidence (%) of Laboratory Abnormalities in Herpes Zoster and Genital Herpes Trial Populations
| Reaction | VALTREX 1 gram 3 Times Daily (n = 967) | Placebo (n = 195) | VALTREX 1 gram Twice Daily (n = 1,194) | VALTREX 500 mg Twice Daily (n = 1,159) | Placebo (n = 439) | VALTREX 1 gram Once Daily (n = 269) | VALTREX 500 mg Once Daily (n = 266) | Placebo (n = 134) |
|---|---|---|---|---|---|---|---|---|
| Hemoglobin | 0.8% | 0% | 0.3% | 0.2% | 0% | 0% | 0.8% | 0.8% |
| White blood cells | 1.3% | 0.6% | 0.7% | 0.6% | 0.2% | 0.7% | 0.8% | 1.5% |
| Platelet count | 1.0% | 1.2% | 0.3% | 0.1% | 0.7% | 0.4% | 1.1% | 1.5% |
| AST (SGOT) | 1.0% | 0% | 1.0% | a | 0.5% | 4.1% | 3.8% | 3.0% |
| Serum creatinine | 0.2% | 0% | 0.7% | 0% | 0% | 0% | 0% | 0% |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following serious adverse reactions are discussed in greater detail in other sections of the labeling: • Thrombotic Thrombocytopenic Purpura/Hemolytic Uremic Syndrome [see Warnings and Precautions ( 5.1 )]. • Acute Renal Failure [see Warnings and Precautions ( 5.2 )]. • Central Nervous System Effects [see Warnings and Precautions ( 5.3 )]. • Severe Cutaneous Adverse Reactions [see Warnings and Precautions ( 5.4 )]. The most common adverse reactions reported in at least 1 indication by greater than 10% of adult subjects treated with VALTREX and observed more frequently with VALTREX compared with placebo are headache, nausea, and abdominal pain. The only adverse reaction reported in greater than 10% of pediatric subjects aged less than 18 years was headache. • The most common adverse reactions reported in at least one indication by greater than 10% of adult subjects treated with VALTREX and more commonly than in subjects treated with placebo are headache, nausea, and abdominal pain. ( 6.1 ) • The only adverse reaction occurring in greater than 10% of pediatric subjects aged less than 18 years was headache. ( 6.2 ) To report SUSPECTED ADVERSE REACTIONS, contact GlaxoSmithKline at 1-888-825-5249 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. …
Source: FDA drug label, GlaxoSmithKline LLC, effective November 12, 2025 (SPL set f8e0d8f8-cb73-4206-a484-88f5c4fbd719), via openFDA; the full label on DailyMed
What do FAERS reports show?
70,054 reports list valacyclovir among the medicines taken, each a report of something that happened, not proof the medicine caused it; 5,662 are coded as a death and 26,096 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 21 reactions below, 10 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Valacyclovir: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions