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Side Effects


FORTAMET® (metformin hcl) Clinical Studies

In the controlled clinical studies of FORTAMET® (metformin hcl) in patients with type 2 diabetes, a total of 424 patients received FORTAMET® (metformin hcl) therapy (up to 2500 mg/day) and 430 patients received immediate-release metformin. Adverse reactions reported in ≥ 5% of the FORTAMET® (metformin hcl) or immediate-release metformin patients are listed in Table 6. These pooled results show that the most frequently reported adverse reactions in the FORTAMET® (metformin hcl) group were inf ection, diarrhea, and nausea. Similar incidences of these adverse reactions were seen in the immediate-release metformin group.

Table 6: Number and Percentage of Patients With the Most Common (Incidence ≥ 5%)Treatment-Emergent Signs or Symptoms by Body System and Preferred Term Pooled Phase II and III Studies

Immediate-Release Metformin
Body System
  Preferred Term
n (%) n (%)
Body as a Whole
  Accidental Injury 31 (7.3) 24 (5.6)
  Headache 20 (4.7) 22 (5.1)
  Infection 87 (20.5) 90 (20.9)
Digestive System
  Diarrhea 71 (16.7) 51 (11.9)
  Dyspepsia 18 (4.2) 22 (5.1)
  Nausea 36 (8.5) 32 (7.4)
Respiratory System
  Rhinitis 18 (4.2) 24 (5.6)

The most frequent adverse events thought to be related to FORTAMET® (metformin hcl) were diarrhea, nausea, dyspepsia, flatulence, and abdominal pain. The frequency of dyspepsia was 4.2% in the FORTAMET® (metformin hcl) group compared to 5.1% in the immediate-release group, the frequency of flatulence was 3.5% in the FORTAMET® (metformin hcl) group compared to 3.7% in the im mediate-release group, and the frequency of abdominal pain was 3.3% in the FORTAMET® (metformin hcl) group compared to 4.4% in the immediate-release group.

In the controlled studies, 4.7% of patients treated with FORTAMET® (metformin hcl) and 4.9% of patients treated with immediate-release metformin were discontinued due to adverse events.

Immediate-Release Metformin

Immediate-Release Metformin Phase III Clinical Studies

In a U.S. double-blind clinical study of immediate-release metformin in patients with type 2 diabetes, a total of 141 patients received immediate-release metformin therapy (up to 2550 mg per day) and 145 patients received placebo. Adverse reactions reported in greater than 5% of the immediate-release metformin patients, and that were more common in immediate-release metformin- than placebo-treated patients, are listed in Table 7.

Table 7: Most Common Adverse Reactions (>5.0%) in a Placebo-Controlled Clinical Study of Immediate-Release Metformin Monotherapy*

  Immediate-Release Metformin Monotherapy
(n = 141)
(n = 145)
Adverse Reaction % 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 immediate-release metformin than placebo-treated patients

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

Pediatric Patients

No pediatric clinical studies have been conducted with FORTAMET® (metformin hcl) . In clinical trials with immediate-release metformin in pediatric patients with type 2 diabetes, the profile of adverse reactions was similar to that observed in adults

Read the Fortamet (metformin hcl) Side Effects Center for a complete guide to possible side effects


Clinical Evaluation of Drug Interactions Conducted with Immediate-Release Metformin

Glyburide - In a single-dose interaction study in type 2 diabetes patients, co-administration of metformin and glyburide did not result in any changes in either metformin pharmacokinetics or pharmacodynamics. Decreases in glyburide AUC and C max were observed, but were highly variable.

The single-dose nature of this study and the lack of correlation between glyburide blood levels and pharmacodynamic effects, makes the clinical significance of this interaction uncertain (see DOSAGE AND ADMINISTRATION: Concomitant FORTAMET® (metformin hcl) and Oral Sulfonylurea Therapy in Adult Patients).

Furosemide - A single-dose, metformin-furosemide drug interaction study in healthy subjects demonstrated that pharmacokinetic parameters of both compounds were affected by co-administration. Furosemide increased the metformin plasma and blood Cmax by 22% and blood AUC by 15%, without any significant change in metformin renal clearance.

When administered with metformin, the Cmax and AUC of furosemide were 31% and 12% smaller, respectively, than when administered alone, and the terminal half-life was decreased by 32%, without any significant change in furosemide renal clearance. No information is available about the interaction of metformin and furosemide when co-administered chronically.

Nifedipine - A single-dose, metformin-nifedipine drug interaction study in normal healthy volunteers demonstrated that co-administration of nifedipine increased plasma metformin C max and AUC by 20% and 9%, respectively, and increased the amount excreted in the urine. Tmax and half-life were unaffected. Nifedipine appears to enhance the absorption of metformin. Metformin had minimal effects on nifedipine.

Cationic drugs - Cationic drugs (e.g., amiloride, digoxin, morphine, procainamide, quinidine, quinine, ranitidine, triamterene, trimethoprim, or vancomycin) that are eliminated by renal tubular secretion theoretically have the potential for interaction with metformin by competing for common renal tubular transport systems. Such interaction between metformin and oral cimetidine has been observed in normal healthy volunteers in both single- and multiple-dose, metformin-cimetidine drug interaction studies, with a 60% increase in peak metformin plasma and whole blood concentrations and a 40% increase in plasma and whole blood metformin AUC. There was no change in elimination half-life in the single-dose study. Metformin had no effect on cimetidine pharmacokinetics. Although such interactions remain theoretical (except for cimetidine), careful patient monitoring and dose adjustment of FORTAMET® (metformin hcl) and/or the interfering drug is recommended in patients who are taking cationic medications that are excreted via the proximal renal tubular secretory system.

Other - Certain drugs tend to produce hyperglycemia and may lead to loss of glycemic control. These drugs include the thiazides and other diuretics, corticosteroids, phenothiazines, thyroid products, estrogens, oral contraceptives, phenytoin, nicotinic acid, sympathomimetics, calcium channel blocking drugs, and isoniazid. When such drugs are administered to a patient receiving FORTAMET® (metformin hcl) , the patient should be closely observed for loss of blood glucose control. When such drugs are withdrawn from a patient receiving FORTAMET® (metformin hcl) , the patient should be observed closely for hypoglycemia.

In healthy volunteers, the pharmacokinetics of metformin and propranolol, and metformin and ibuprofen were not affected when co-administered in single-dose interaction studies.

Metformin is negligibly bound to plasma proteins and is, therefore, less likely to interact with highly protein-bound drugs such as salicylates, sulfonamides, chloramphenicol, and probenecid, as compared to the sulfonylureas, which are extensively bound to serum proteins.

Read the Fortamet Drug Interactions Center for a complete guide to possible interactions

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 9/25/2008

Side Effects

Fortamet - User Reviews

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