Metronidazole, an antibiotic and antiprotozoal medication sold under brand names like Flagyl, is used to treat a wide array of infections [1.6.5, 1.6.6]. It is effective against anaerobic bacteria and certain parasites, treating conditions in the skin, stomach, vagina, joints, and nervous system [1.3.3, 1.3.4, 1.3.5]. While generally well-tolerated, it is associated with several potential side effects, ranging from common and mild to rare and serious [1.4.3].
Understanding Metronidazole and Its Uses
Metronidazole works by entering the bacterial or parasitic cell and breaking down its DNA, which stops its growth and leads to cell death [1.6.2]. It is prescribed for numerous infections, including:
- Bacterial vaginosis [1.3.2]
- Trichomoniasis (a sexually transmitted infection) [1.3.2, 1.3.3]
- Pelvic inflammatory disease [1.6.5]
- Infections in the abdomen, liver, skin, heart, and brain [1.3.5]
- C. difficile infection of the colon [1.3.5]
The medication is available in several forms, including oral tablets, capsules, liquids, topical gels, and intravenous injections [1.2.3, 1.6.5].
Most Common Side Effects
The most frequently reported side effects of oral metronidazole are often gastrointestinal and typically temporary [1.2.3, 1.4.3].
Gastrointestinal Discomfort
- Nausea: Occurs in up to 12% of people taking the drug [1.4.3]. Taking the medication with a meal or snack can help lessen this effect [1.4.3, 1.6.6].
- Headache: This is the most likely side effect, affecting up to 18% of users [1.4.3, 1.3.2].
- Metallic Taste: A sharp, unpleasant metallic taste is a well-known side effect, reported in up to 9% of people [1.4.3, 1.2.1]. This taste usually resolves after treatment ends [1.4.3].
- Diarrhea: Like many antibiotics, metronidazole can cause diarrhea, affecting up to 4% of individuals [1.4.3].
- Other GI Issues: Other common effects include stomach cramps, upset stomach, constipation, and loss of appetite [1.2.1].
Other Common Reactions
- Vaginitis: Inflammation of the vagina can occur in up to 15% of patients [1.8.6].
- Darkened Urine: Metronidazole can cause urine to appear darker in color. This is considered a harmless side effect caused by a metabolite of the drug [1.2.3, 1.3.5].
- Mouth and Tongue Irritation: Some people may experience a furry tongue or irritation in the mouth [1.2.1].
Serious Side Effects and When to Contact a Doctor
While less common, metronidazole can cause serious side effects that require immediate medical attention [1.4.2].
Neurological Effects
Rare but serious neurological problems can occur, particularly with prolonged use (longer than 2 weeks) [1.4.3]. These are typically reversible after stopping the medication [1.4.3]. Symptoms to watch for include:
- Peripheral Neuropathy: Numbness, tingling, pain, or burning in the hands or feet [1.4.2, 1.2.4].
- Seizures: Any convulsive activity should be reported immediately [1.4.2].
- Aseptic Meningitis: Symptoms include fever, stiff neck, headache, confusion, and sensitivity to light [1.2.4, 1.4.4].
- Encephalopathy: This brain disorder can cause confusion, dizziness, and difficulty with speech, coordination, or balance [1.4.3].
Other Serious Reactions
- Severe Skin Reactions: Seek immediate help for blistering, peeling skin, a red skin rash, or sores in the mouth, as these could be signs of conditions like Stevens-Johnson syndrome [1.2.2, 1.4.1].
- Liver Problems: Though rare, life-threatening liver issues can occur, especially in patients with Cockayne syndrome [1.2.2, 1.2.4]. Symptoms include yellowing of the skin or eyes (jaundice), dark urine, upper right stomach pain, and nausea [1.4.4].
- Allergic Reactions: Signs of a severe allergic reaction include hives, itching, swelling of the face, lips, or throat, and difficulty breathing [1.4.4].
| Side Effect Category | Common & Mild Examples | Serious & Rare Examples |
|---|---|---|
| Gastrointestinal | Nausea, headache, metallic taste, diarrhea, stomach cramps [1.2.1, 1.4.3] | Severe stomach pain radiating to the back (pancreatitis) [1.3.7] |
| Neurological | Dizziness, headache [1.2.1] | Seizures, numbness/tingling in limbs, confusion, stiff neck [1.4.2, 1.4.4] |
| Dermatological | Mild rash, itching [1.2.3] | Blistering or peeling skin (Stevens-Johnson syndrome) [1.2.2] |
| Systemic | Darkened urine, loss of appetite [1.3.5] | Yellowing skin/eyes (jaundice), signs of new infection (fever, sore throat) [1.4.4, 1.2.1] |
Important Precautions: Alcohol and Drug Interactions
The Alcohol Interaction
It is critical to avoid consuming alcoholic beverages and any products containing alcohol or propylene glycol while taking metronidazole and for at least three days after the final dose [1.2.1, 1.3.3]. The combination can cause a severe disulfiram-like reaction with symptoms including [1.5.3, 1.5.4]:
- Severe nausea and vomiting
- Stomach cramps
- Headaches
- Flushing (redness of the face)
- Pounding heartbeat (palpitations)
Drug Interactions
Metronidazole can interact with several other medications. It is crucial to inform a healthcare provider of all medications being taken, including over-the-counter drugs and supplements [1.2.2]. Key interactions include:
- Warfarin (blood thinners): Metronidazole can increase the effects of warfarin, raising the risk of bleeding [1.7.1].
- Lithium: Levels of lithium can be increased, leading to a higher risk of toxicity [1.7.1].
- Disulfiram: This combination is contraindicated and can cause psychotic reactions [1.7.1, 1.7.2]. A two-week gap is needed between taking these drugs [1.7.1].
- Busulfan (cancer medication): Metronidazole can increase busulfan levels, leading to toxicity [1.7.2].
Conclusion
Metronidazole is an effective antibiotic for a variety of infections, but it comes with a range of potential side effects. The most common are gastrointestinal issues and a metallic taste, which are usually manageable and temporary [1.2.5]. However, users should be aware of the potential for more serious neurological, skin, and liver reactions and seek immediate medical care if they occur [1.4.2, 1.4.4]. The strict avoidance of alcohol is a critical precaution to prevent a severe reaction [1.5.3]. Always take metronidazole exactly as prescribed and complete the full course of treatment to ensure the infection is fully resolved and to prevent antibiotic resistance [1.3.6].
For more detailed information, consult the patient information leaflet or speak with a healthcare provider. An authoritative source for drug information is MedlinePlus.
