Metformin vs Tirzepatide: side effects in FDA adverse event reports
At a glance
| Metformin | Tirzepatide | |
|---|---|---|
| Drug class | – | Glucose-dependent Insulinotropic Polypeptide Receptor Agonist, GLP-1 Receptor Agonist |
| Brand names | Metformin Hydrochloride | Mounjaro |
| FAERS reports | 354,745 | 160,620 |
| Serious | 69% | 18% |
| Death outcome | 29,950 | 889 |
| Boxed warning | Yes LACTIC ACIDOSIS Postmarketing cases of metformin-associated lactic acidosis have resulted in death, hypothermia, hypotension, and resistant bradyarrhythmias. Th | Yes RISK OF THYROID C-CELL TUMORS In both male and female rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinical |
Signals both drugs share
| Reaction | Metformin | Tirzepatide | Label screen (A / B) |
|---|---|---|---|
| Impaired gastric emptying | 1,751 · PRR 9.3 | 1,678 · PRR 19.7 | Not in label — unassessed / Labeled (related wording) |
| Gastrointestinal hypomotility | 384 · PRR 12.1 | 367 · PRR 25.6 | Not in label — unassessed / Not in label — unassessed |
Labeled signals distinctive to Metformin
- Acute kidney injury — n=17,231, PRR 7.4 Labeled — boxed warning
- Lactic acidosis — n=17,141, PRR 67.6 Labeled — boxed warning
- Hypoglycaemia — n=7,287, PRR 10.3 Labeled — boxed warning
- Metabolic acidosis — n=6,200, PRR 14.2 Labeled adverse reaction
- Diabetic ketoacidosis — n=4,053, PRR 11.9 Labeled adverse reaction
- Diabetes mellitus inadequate control — n=2,895, PRR 11.4 Labeled (related wording)
- Euglycaemic diabetic ketoacidosis — n=1,754, PRR 46.1 Labeled (related wording)
- Ketoacidosis — n=1,347, PRR 12.8 Labeled adverse reaction
Labeled signals distinctive to Tirzepatide
- Injection site haemorrhage — n=5,281, PRR 9.2 Labeled (related wording)
- Eructation — n=2,913, PRR 23.5 Labeled adverse reaction
- Injection site rash — n=1,728, PRR 7.8 Labeled (related wording)
- Injection site injury — n=1,463, PRR 47.3 Labeled (related wording)
- Increased appetite — n=1,113, PRR 8.2 Labeled (related wording)
- Injection site hypersensitivity — n=548, PRR 28.2 Labeled (related wording)
- Injection site coldness — n=303, PRR 74.3 Labeled (related wording)
- Injection site paraesthesia — n=165, PRR 15.9 Labeled (related wording)
Common questions
Which has more reported side effects, Metformin or Tirzepatide?
Metformin has 354,745 FAERS reports and Tirzepatide 160,620. More reports does not mean more dangerous: volume follows how widely and how long a drug has been used, prescription vs over-the-counter status, and publicity. The seriousness share (69% vs 18%) is a better like-for-like signal of the kind of problems reported.
Which side effects do Metformin and Tirzepatide share?
Both drugs carry disproportionality signals for impaired gastric emptying, gastrointestinal hypomotility. Shared signals often reflect a shared drug class or the condition both are used for.
Is Metformin or Tirzepatide safer?
FAERS cannot answer that: it has no denominator and no comparator. Use this page to see which risks are already on each label and which signals are unlabeled, then check the prescribing information and your clinician for your situation.
Sources: openFDA FAERS pair statistics (OpenPV build 2026-09-14), FDA labels via openFDA, openFDA count queries for seriousness and deaths. Pair chosen because it is a commonly searched comparison. Generated 2026-10-08.