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Flagyl 375 Side Effects

Generic Name: metronidazole

Note: This page contains information about the side effects of metronidazole. Some of the dosage forms included on this document may not apply to the brand name Flagyl 375.

For the Consumer

Applies to metronidazole: oral capsule, oral powder for suspension, oral suspension, oral tablet, oral tablet extended release

In addition to its needed effects, some unwanted effects may be caused by metronidazole (the active ingredient contained in Flagyl 375). In the event that any of these side effects do occur, they may require medical attention.

Major Side Effects

You should check with your doctor immediately if any of these side effects occur when taking metronidazole:

More common:
  • Agitation
  • back pain
  • blindness
  • blurred vision
  • burning, numbness, tingling, or painful sensations in the hands or feet
  • changes in speech patterns
  • confusion
  • convulsions
  • decreased vision
  • depression
  • dizziness
  • drowsiness
  • eye pain
  • fever
  • hallucinations
  • headache
  • irritability
  • lack of coordination
  • nausea
  • seizures
  • shakiness and unsteady walk
  • slurred speech
  • stiff neck or back
  • trouble speaking
  • unsteadiness, trembling, or other problems with muscle control or coordination
  • unusual tiredness or weakness
  • vomiting
  • weakness in the arms, hands, legs, or feet
Less common:
  • Black, tarry stools
  • blood in the urine or stools
  • body aches or pain
  • chills
  • clumsiness or unsteadiness
  • difficulty with breathing
  • ear congestion
  • feeling of pelvic pressure
  • frequent or painful urination
  • loss of voice
  • nasal congestion
  • pinpoint red spots on the skin
  • runny nose
  • skin rash, hives, redness, or itching
  • sneezing
  • stomach and back pain (severe)
  • unusual bleeding or bruising
  • vaginal irritation, discharge, or dryness not present before taking the medicine
  • Bleeding gums
  • bloating
  • chest pain
  • constipation
  • cough
  • dark-colored urine
  • fast heartbeat
  • indigestion
  • loss of appetite
  • painful or difficult urination
  • pains in the stomach, side, or abdomen, possibly radiating to the back
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • swollen glands
  • yellow eyes or skin
Incidence not known:
  • Blistering, peeling, or loosening of the skin
  • bloody or cloudy urine
  • burning while urinating
  • continuing diarrhea
  • continuing stomach pain
  • diarrhea
  • feeling of warmth
  • increased volume of pale, dilute urine
  • joint or muscle pain
  • loss of bladder control
  • red skin lesions, often with a purple center
  • red, irritated eyes
  • redness of the face, neck, arms, and occasionally, upper chest
  • redness of the skin

Minor Side Effects

Some of the side effects that can occur with metronidazole may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

More common:
  • Abdominal or stomach cramps
  • dizziness or lightheadedness
  • feeling of constant movement of self or surroundings
  • heartburn
  • sensation of spinning
  • trouble sleeping
  • weight loss
Less common or rare:
  • Change in taste sensation
  • congestion
  • dry mouth
  • pain or tenderness around the eyes and cheekbones
  • tender, swollen glands in the neck
  • trouble with swallowing
  • unpleasant or sharp metallic taste
  • voice changes
Incidence not known:
  • Decreased interest in sexual intercourse
  • inability to have or keep an erection
  • loss in sexual ability, desire, drive, or performance
  • painful sexual intercourse

For Healthcare Professionals

Applies to metronidazole: compounding powder, intravenous powder for injection, intravenous solution, oral capsule, oral suspension, oral tablet, oral tablet extended release


The most serious side effects reported with metronidazole (the active ingredient contained in Flagyl 375) have included convulsive seizures, encephalopathy, aseptic meningitis, optic and peripheral neuropathy (primarily characterized by numbness or paresthesia of an extremity). Persistent peripheral neuropathy has been reported with prolonged oral administration of metronidazole.[Ref]

Nervous system

Headache (18%) and dizziness (4%) were reported in patients using the extended-release tablet formulation. Headaches were considered severe in 10% of these patients.

A rare but serious side effect of metronidazole (the active ingredient contained in Flagyl 375) was peripheral neuropathy which was thought to be related to dose and duration of therapy. Most cases occurred after doses of 1000 to 2400 mg per day for at least 30 days, or a total dose of 50 grams. Neuropathy was generally located in the lower extremities and presented with numbness, tingling, and paresthesias. Patients should be advised to discontinue metronidazole if they experience these effects. Neuropathy may be persistent despite discontinuation of metronidazole. Seizures associated with metronidazole were rare, but did occur.

Several cases of MRI abnormalities of the dentate nuclei associated with symptoms of ataxia and dysarthria have been reported. The symptoms and dentate nuclei lesions resolved several weeks after discontinuation of metronidazole. These symptoms and metronidazole-induced lesions in the brain stem and cerebellar nuclei have also been described in rat studies.[Ref]

Very common (10% or more): Headache
Common (1% to 10%): Dizziness
Frequency not reported: Encephalopathy, aseptic meningitis, peripheral neuropathy (characterized mainly by numbness or paresthesia of an extremity), convulsive seizures, vertigo, syncope, incoordination, ataxia, confusion, dysarthria, weakness, reversible abnormalities in the cerebellar dentate nuclei[Ref]


Very common (10% or more): Nausea
Common (1% to 10%): Taste perversion (metallic taste), abdominal pain, diarrhea, dry mouth
Rare (less than 0.1%): Pseudomembranous colitis, pancreatitis
Frequency not reported: Sharp unpleasant metallic taste, vomiting, abdominal discomfort, epigastric distress, abdominal cramping, constipation, proctitis, coating of the tongue (furry tongue), glossitis, stomatitis, taste of alcoholic beverages modified[Ref]

Nausea (10%), taste perversion (metallic taste; 9%), abdominal pain (4%), diarrhea (4%), and dry mouth (2%) in patients using the extended-release tablet formulation. Nausea was considered severe in less than 2% of these patients.

Furry tongue, glossitis, and stomatitis may be associated with sudden overgrowth of Candida.

Although metronidazole has been used to treat pseudomembranous colitis, it has rarely also been implicated as a causative agent. These rare cases have been successfully treated with vancomycin.[Ref]


Very common (10% or more): Vaginitis
Common (1% to 10%): Genital pruritus, abnormal urine, dysmenorrhea, urinary tract infection
Rare (less than 0.1%): Darkened urine
Frequency not reported: Dysuria, cystitis, polyuria, incontinence, vulvovaginal candidiasis, vaginal itching, dyspareunia, sense of pelvic pressure[Ref]

Vaginitis (15%), genital pruritus (5%), abnormal urine (3%), dysmenorrhea (3%), and urinary tract infection (2%) were reported in patients using the extended-release tablet formulation.

Instances of darkened urine have been investigated. The pigment responsible has not been positively identified; however, it is most likely a metabolite of metronidazole and appears to have no clinical significance.[Ref]


Bacterial infection (7%), influenza-like symptoms (6%), and moniliasis (3%) were reported in patients using the extended-release tablet formulation.

A disulfiram-like reaction has been reported in some patients drinking ethanol while taking metronidazole (the active ingredient contained in Flagyl 375) Patients have experienced abdominal distress, vomiting, flushing, nausea, headache, and hypotension. Patients should be advised to refrain from all forms of ethanol while taking metronidazole and for at least 72 hours after the last dose.[Ref]

Common (1% to 10%): Bacterial infection, influenza-like symptoms, moniliasis
Frequency not reported: Fever, overgrowth of Candida, disulfiram-like reaction (abdominal distress, nausea, vomiting, flushing, hypotension, or headache) with ethanol[Ref]


Upper respiratory tract infection (4%), rhinitis (4%), sinusitis (3%), and pharyngitis (3%) were reported in patients using the extended-release tablet formulation.

Common (1% to 10%): Upper respiratory tract infection, rhinitis, sinusitis, pharyngitis


Metronidazole (the active ingredient contained in Flagyl 375) has rarely been associated with a serum sickness-like reaction which presented as arthralgias, malaise, fever, chills, pruritus, and rash.[Ref]

Rare (less than 0.1%): Serum sickness-like reaction
Frequency not reported: Hypersensitivity (including urticaria, erythematous rash, Stevens-Johnson syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth [or vagina or vulva], fever), fixed drug eruptions, cross-sensitivity to other nitroimidazole derivatives[Ref]


Rare (less than 0.1%): Reversible thrombocytopenia
Frequency not reported: Reversible neutropenia (leukopenia)[Ref]


Frequency not reported: Flattening of the T-wave in ECG tracings


Frequency not reported: Erythematous rash, pruritus


Frequency not reported: Thrombophlebitis (after IV infusion)

Thrombophlebitis can be minimized or avoided by avoiding prolonged use of indwelling IV catheters.


Frequency not reported: Irritability, depression, decreased libido, insomnia, disorientation, agitation, hallucinations


Frequency not reported: Optic neuropathy


Frequency not reported: Anorexia


Frequency not reported: Fleeting joint pains (sometimes resembling "serum sickness")


Breast and colon cancer have been reported in Crohn's disease patients treated with high doses of metronidazole (the active ingredient contained in Flagyl 375) for extended periods of time.

Frequency not reported: Breast cancer, colon cancer


1. Rosenblatt JE, Edson RS "Metronidazole." Mayo Clin Proc 62 (1987): 1013-7

2. Koch-Weser J "Metronidazole." N Engl J Med 303 (1980): 1212-8

3. Stahlberg D, Barany F, Einarsson K, Ursing B, Elmquist D, Persson A "Neurophysiologic studies of patients with Crohn's disease on long-term treatment with metronidazole." Scand J Gastroenterol 26 (1991): 219-24

4. Wienbren M, Perinpanayagam RM, Camba L, Lee CA "Convulsions and encephalopathy in a patient with leukaemia after treatment with metronidazole." J Clin Pathol 38 (1985): 1076

5. Learned-Coughlin S "Peripheral neuropathy induced by metronidazole." Ann Pharmacother 28 (1994): 536

6. Lawford R, Sorrell TC "Amebic abscess of the spleen complicated by metronidazole-induced neurotoxicity: case report." Clin Infect Dis 19 (1994): 346-8

7. Beloosesky Y, Grosman B, Marmelstein V, Grinblat J "Convulsions induced by metronidazole treatment for Clostridium difficile-associated disease in chronic renal failure." Am J Med Sci 319 (2000): 338-9

8. Boyce EG, Cookson ET, Bond WS "Persistent metronidazole-induced peripheral neuropathy." DICP 24 (1990): 19-21

9. Juillerat P, Pittet V, Felley C, et al. "Drug safety in Crohn's disease therapy." Digestion 76 (2007): 161-8

10. Ahmed A, Laes DJ, Bressler EL "Reversible magnetic resonance imaging findings in metronidazole-induced encephalopathy." Neurology 45 (1995): 588-9

11. Schentag JJ, Ziemniak JA, Greco JM, Rainstein M, Buckley RJ "Mental confusion in a patient treated with metronidazole: a concentration-related effect." Pharmacotherapy 2 (1982): 384-7

12. Woodruff BK, Wijdicks EF, Marshall WF, et al. "Reversible metronidazole-induced lesions of the cerebellar nuclei." N Engl J Med 346 (2002): 68-9

13. Kusumi RK, Plouffe JF, Wyatt RH, Fass RJ "Central nervous sytem toxicity associated with metronidazole therapy." Ann Intern Med 93 (1980): 59-60

14. Finegold SM "Therapy for infections due to anaerobic bacteria: an overview." J Infect Dis 135 Suppl (1977): S25-9

15. Whyte CA, Shivdat-Nanhoe R, Kramer P "A Case of Amoxicillin-Induced Meningitis." Clin Infect Dis 46 (2008): 642

16. Alvarez RS, Richardson DA, Bent AE, Ostergard DR "Central nervous system toxicity related to prolonged metronidazole therapy." Am J Obstet Gynecol 145 (1983): 640-1

17. Bonkowsky JL, Sondrup C, Benedict SL "Acute reversible cerebellar lesions associated with metronidazole therapy." Neurology 68 (2007): 180

18. Schreiber W, Spernal J "Metronidazole-induced psychotic disorder." Am J Psychiatry 154 (1997): 1170-1

19. Gerding DN, Muto CA, Owens RC Jr "Treatment of Clostridium difficile infection." Clin Infect Dis 46 Suppl 1 (2008): S32-42

20. Duffy LF, Daum F, Fisher SE, et al "Peripheral neuropathy in Crohn's disease patients treated with metronidazole." Gastroenterology 88 (1985): 681-4

21. Schwebke JR, Desmond RA "A Randomized Trial of the Duration of Therapy with Metronidazole plus or minus Azithromycin for Treatment of Symptomatic Bacterial Vaginosis." Clin Infect Dis 44 (2007): 213-9

22. Sura ME, Heinrich KA, Suseno M "Metronidazole-associated pancreatitis." Ann Pharmacother 34 (2000): 1152-5

23. Saginur R, Hawley CR, Bartlett JG "Colitis associated with metronidazole therapy." J Infect Dis 141 (1980): 772-4

24. Eland IA, van Puijenbroek EP, Sturkenboom MJ, Wilson JH, Stricker BH "Drug-associated acute pancreatitis: twenty-one years of spontaneous reporting in The Netherlands." Am J Gastroenterol 94 (1999): 2417-22

25. Friedman GD, Selby JV "How often does metronidazole induce pancreatitis." Gastroenterology 98 (1990): 1702-3

26. Lagrotteria D, Holmes S, Smieja M, Smaill F, Lee C "Prospective, Randomized Inpatient Study of Oral Metronidazole versus Oral Metronidazole and Rifampin for Treatment of Primary Episode of Clostridium difficile-Associated Diarrhea." Clin Infect Dis 43 (2006): 547-52

27. Loulergue P, Mir O "Metronidazole-induced pancreatitis during HIV infection." AIDS 22 (2008): 545-6

28. Daly JJ, Chowdary KV "Pseudomembranous colitis secondary to metronidazole." Dig Dis Sci 28 (1983): 573-4

29. Bingley PJ, Harding GM "Clostridium difficile colitis following treatment with metronidazole and vancomycin." Postgrad Med J 63 (1987): 993-4

30. Giannini AJ, DeFrance DT "Metronidazole and alcohol: potential for combinative abuse." J Toxicol Clin Toxicol 20 (1983): 509-15

31. Harries DP, Teale KF, Sunderland G "Metronidazole and alcohol: potential problems." Scott Med J 35 (1990): 179-80

32. Alexander I "Alcohol-antabuse syndrome in patients receiving metronidazole during gynaecological treatment." Br J Clin Pract 39 (1985): 292-3

33. Shelley WB, Shelley ED "Fixed drug eruption due to metronidazole." Cutis 39 (1987): 393-4

34. Helms DJ, Mosure DJ, Secor WE, Workowski KA "Management of Trichomonas vaginalis in women with suspected metronidazole hypersensitivity." Am J Obstet Gynecol 198 (2008): 370. e1-7.

35. Knowles S, Choudhury T, Shear NH "Metronidazole hypersensitivity." Ann Pharmacother 28 (1994): 325-6

36. Hermida MD, Consalvo L, Lapadula MM, Della Giovanna P, Cabrera HN "Bullous fixed drug eruption induced by intravaginal metronidazole ovules, with positive topical provocation test findings." Arch Dermatol 147 (2011): 250-1

37. Mishra D, Mobashir M, Zaheer MS "Fixed drug eruption and cross-reactivity between tinidazole and metronidazole." Int J Dermatol 29 (1990): 740

38. White CM, Price JJ, Hunt KM "Bone marrow aplasia associated with metronidazole." Br Med J 280 (1980): 647

39. Weart CW, Hyman LC "Serum sickness associated with metronidazole." South Med J 76 (1983): 410-1

40. Smith JA "Neutropenia associated with metronidazole therapy." Can Med Assoc J 123 (1980): 202

Not all side effects for Flagyl 375 may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

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