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Metformin (Metformin Hydrochloride) - Side Effects and Adverse Reactions



In a US double-blind clinical study of Metformin HCl in patients with type 2 diabetes, a total of 141 patients received Metformin HCl therapy (up to 2550 mg per day) and 145 patients received placebo. Adverse reactions reported in greater than 5% of the Metformin HCl patients, and that were more common in Metformin HCl - than placebo-treated patients, are listed in Table 9.

Table 9:    Most Common Adverse Reactions (>5.0 Percent) in a Placebo-Controlled

                 Clinical Study of Metformin HCl Monotherapy*

Adverse Reaction Metformin HCl Monotherapy
% of Patients
 Diarrhea  53.2  11.7
 Nausea/Vomiting  25.5  8.3
 Flatulence  12.1  5.5
 Asthenia  9.2  5.5
 Indigestion  7.1  4.1
 Abdominal Discomfort  6.4  4.8
 Headache  5.7  4.8

* Reactions that were more common in Metformin HCl -than placebo-treated patients.

Diarrhea led to discontinuation of study medication in 6% of patients treated with Metformin HCl. Additionally, the following adverse reactions were reported in ≥ 1.0 to ≤ 5.0% of Metformin HCl patients and were more commonly reported with Metformin HCl than placebo: abnormal stools, hypoglycemia, myalgia, lightheaded, dyspnea, nail disorder, rash, sweating increased, taste disorder, chest discomfort, chills, flu syndrome, flushing, palpitation.

Pediatric Patients

In clinical trials with Metformin HCl in pediatric patients with type 2 diabetes, the profile of adverse reactions was similar to that observed in adults.


Below is a sample of reports where side effects / adverse reactions may be related to Metformin. The information is not vetted and should not be considered as verified clinical evidence.

Possible Metformin side effects / adverse reactions in 69 year old male

Reported by a physician from United Kingdom on 2011-10-27

Patient: 69 year old male

Reactions: Skin Discolouration, Skin Lesion, Rash Erythematous, Rash Macular, Naevus Flammeus

Suspect drug(s):
    Indication: Type 2 Diabetes Mellitus

    Indication: Type 2 Diabetes Mellitus

    Administration route: Oral
    Indication: Hypercholesterolaemia

    Indication: Type 2 Diabetes Mellitus

Possible Metformin side effects / adverse reactions in 50 year old male

Reported by a health professional (non-physician/pharmacist) from United States on 2011-10-28

Patient: 50 year old male

Reactions: Drug Level Increased, Intentional Overdose, Urine Output Decreased, Hypotension, Mental Status Changes, Lactic Acidosis, Toxicity TO Various Agents, Suicide Attempt, Anuria, Renal Failure Acute

Adverse event resulted in: hospitalization

Suspect drug(s):
Metformin HCL
    Dosage: 500 mg, bid
    Indication: Type 2 Diabetes Mellitus

    Dosage: 52 g, unk

Possible Metformin side effects / adverse reactions in 66 year old male

Reported by a physician from Germany on 2011-11-08

Patient: 66 year old male weighing 160.0 kg (352.0 pounds)

Reactions: Bile Duct Cancer

Adverse event resulted in: death

Suspect drug(s):
    Administration route: Oral
    Indication: Type 2 Diabetes Mellitus
    Start date: 2010-10-01
    End date: 2011-09-01

    Administration route: Oral
    Indication: Type 2 Diabetes Mellitus
    Start date: 2009-05-01
    End date: 2011-09-01

See index of all Metformin side effect reports >>

Drug label data at the top of this Page last updated: 2013-09-16

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