Drug side effects
allopurinol Side Effects
The reactions most often reported to the FDA for allopurinol (also sold as Zyloprim), with demographics, serious outcomes, and reporting trends.
Data updated 2026-05-15
- 150,998
- FAERS reports
Data from the FDA Adverse Event Reporting System (FAERS), 1997 to 2025. A FAERS report records a temporal association, not proof of cause. See our methodology for how figures are compiled and de-duplicated.
The most commonly reported side effects of allopurinol come from 150,998 reports to the FDA, broken down here by frequency, patient demographics, and serious outcomes.
This page analyzes 150,998 reports to the FDA Adverse Event Reporting System (FAERS) for allopurinol (Zyloprim): the reactions reported most often, who reported them, serious outcomes such as hospitalization, and year-over-year trends. A FAERS report does not prove the drug caused the effect, and report volume tracks how widely a drug is used, not how likely an effect is for any one person. Educational information, not medical advice.
Total Reports
150,998
Reports Mentioning Death
21,323
14.1% of reports — not proof the medicine caused the death
Hospitalizations
74,277
49.2% of reports
Top Indication
Product Used For Unknown Indication
Most Reported Adverse Reactions
Who Reports Side Effects
Gender Distribution
Age Distribution
Reporting Trend by Year
Reactions in Death Reports
Top reactions reported in 21,323 reports where death was an outcome. These are voluntarily reported and do not establish causation.
| Reaction | Reports |
|---|---|
| DEATH | 6,649 |
| PNEUMONIA | 1,639 |
| SEPSIS | 1,505 |
| DYSPNOEA | 1,204 |
| RENAL FAILURE | 1,204 |
| PYREXIA | 1,042 |
| ACUTE KIDNEY INJURY | 1,029 |
| ANAEMIA | 1,002 |
| GENERAL PHYSICAL HEALTH DETERIORATION | 976 |
| DIARRHOEA | 949 |
| NAUSEA | 893 |
| CARDIAC ARREST | 842 |
| MULTIPLE ORGAN DYSFUNCTION SYNDROME | 825 |
| ASTHENIA | 811 |
| RESPIRATORY FAILURE | 804 |
| HYPOTENSION | 787 |
| THROMBOCYTOPENIA | 748 |
| FATIGUE | 743 |
| VOMITING | 727 |
Reactions in Hospitalization Reports
Top reactions in 74,277 reports where hospitalization was an outcome.
| Reaction | Reports |
|---|---|
| DYSPNOEA | 4,959 |
| PNEUMONIA | 4,591 |
| ACUTE KIDNEY INJURY | 4,575 |
| PYREXIA | 4,538 |
| DIARRHOEA | 4,154 |
| ANAEMIA | 3,921 |
| NAUSEA | 3,461 |
| FATIGUE | 3,365 |
| ASTHENIA | 3,265 |
| FALL | 3,173 |
| HYPOTENSION | 3,021 |
| VOMITING | 2,774 |
| FEBRILE NEUTROPENIA | 2,694 |
| RENAL FAILURE | 2,572 |
| DIZZINESS | 2,450 |
| GENERAL PHYSICAL HEALTH DETERIORATION | 2,367 |
| SEPSIS | 2,367 |
| DEHYDRATION | 2,279 |
| ATRIAL FIBRILLATION | 2,164 |
Nearby, Related Medications
What the FAERS Data Reveals About allopurinol Side Effects
The FDA Adverse Event Reporting System (FAERS) contains 150,998 voluntary reports linked to allopurinol and its brand equivalents (Zyloprim), spanning 1997 through 2025. Of those, 21,323 (14.1%) listed death as an outcome and 74,277 (49.2%) involved hospitalization. The most common indication reported alongside adverse events was Product Used For Unknown Indication.
Demographic breakdowns help contextualize who is being exposed to the drug. Of reports with known sex, 35% were female and 64% male; age distribution skews toward 75+, with 38,718 reports in that bracket. The single most reported reaction is diarrhoea with 8,423 submissions, followed by fatigue and dyspnoea.
How to read these numbers
FAERS is a signal-detection tool, not a scorecard. Reports are voluntary, and a single case may list multiple suspect drugs, so numbers above should not be read as incidence rates or per-patient risk. Widely prescribed drugs naturally accumulate more reports than niche therapies even when individual risk is low. These aggregates are useful for spotting patterns that merit further pharmacovigilance, not for choosing between medications. This page is for educational reference only and is not medical advice, speak with a licensed clinician about any side-effect concerns.
Drugs With a Similar FAERS Report Volume
Other medications with a total FDA adverse-event report count closest to allopurinol's 150,998.
Read our methodology - how this data is sourced, computed, and verified.