Metronidazole

What Is Metronidazole and What Is It Used For?

Quick Answer: Metronidazole is a nitroimidazole antimicrobial used topically to treat the redness and pustules of rosacea, and systemically (orally or intravenously) to treat anaerobic bacterial infections, protozoal infections, and certain gastrointestinal conditions.

Metronidazole was first developed in France in 1960 and quickly became one of the most important antimicrobial agents in clinical medicine. It belongs to the nitroimidazole class of drugs and works by disrupting the DNA of susceptible microorganisms. The drug is particularly effective against organisms that thrive in low-oxygen environments, making it indispensable for treating anaerobic infections throughout the body.

In its topical formulation—available as a 0.75% gel (such as Rozex) or 1% cream (such as Rosazol)—metronidazole is one of the most commonly prescribed treatments for rosacea, a chronic inflammatory skin condition affecting the central face. The topical preparation reduces the redness, papules (bumps), and pustules characteristic of rosacea. Interestingly, the exact mechanism by which metronidazole improves rosacea is not fully understood; it may involve anti-inflammatory and immunosuppressive effects in addition to its antimicrobial properties. A diagnosis of rosacea should be established by a healthcare professional before treatment with topical metronidazole is started.

Systemically, metronidazole is used to treat a wide range of infections caused by anaerobic bacteria and certain protozoa. Its clinical indications include:

• Bacterial vaginosis – one of the first-line treatments recommended by the CDC and WHO
• Trichomoniasis – a sexually transmitted protozoal infection caused by Trichomonas vaginalis
• Amoebiasis – intestinal and hepatic infections caused by Entamoeba histolytica
• Giardiasis – an intestinal parasitic infection caused by Giardia lamblia
• Clostridioides difficile infection (CDI) – used for mild-to-moderate cases, though vancomycin or fidaxomicin are now preferred for most episodes per IDSA/SHEA guidelines
• Intra-abdominal infections – as part of combination therapy for peritonitis, abscesses, and post-surgical infections
• Dental and oral infections – periodontal disease, dental abscesses, and pericoronitis
• Helicobacter pylori eradication – as part of triple or quadruple therapy for peptic ulcer disease
• Pelvic inflammatory disease – in combination with other antibiotics
• Surgical prophylaxis – for colorectal and gynaecological procedures involving anaerobic risk

Metronidazole is available worldwide under numerous brand names including Rozex, Rosazol, Zidoval, Metronidazole Braun, Metronidazol Actavis, Flagyl, and MetroGel. It is available as a generic medication and is included on the World Health Organization’s Model List of Essential Medicines, recognizing it as one of the most efficacious, safe, and cost-effective medicines needed in a health system. The intravenous formulation (5 mg/ml) is reserved for serious infections where oral administration is not feasible.

What Should You Know Before Taking Metronidazole?

Quick Answer: Before using metronidazole, inform your doctor about any allergies, blood disorders, liver disease, or neurological conditions. Avoid alcohol during treatment. Use during pregnancy requires careful medical assessment.

Contraindications

Metronidazole should not be used if you are allergic (hypersensitive) to metronidazole or any other nitroimidazole derivative. Allergic reactions, though rare, can include skin rash, itching, swelling, or in severe cases anaphylaxis. If you have experienced an allergic reaction to any nitroimidazole drug in the past, you should inform your healthcare provider before starting treatment.

For topical formulations, the inactive ingredients (excipients) may also cause hypersensitivity. Common excipients in metronidazole gel include methylparaben (E218), propylparaben (E216), and propylene glycol (E1520). Parabens can cause allergic reactions (potentially delayed), and propylene glycol may cause skin irritation in sensitive individuals. Review the full ingredient list with your pharmacist if you have known sensitivities to preservatives or excipients.

Warnings and Precautions

Several important precautions should be observed when using metronidazole in any formulation:

• Avoid contact with eyes and mucous membranes when applying topical metronidazole. If accidental contact occurs, rinse thoroughly with water.
• Skin irritation: If the skin becomes irritated during topical use, reduce the frequency of application or temporarily discontinue treatment. Contact your doctor if severe irritation develops.
• UV exposure: Avoid excessive sun exposure, sunbeds, and UV lamps during topical treatment, as metronidazole may increase photosensitivity.
• Blood disorders: Inform your doctor if you have a history of any blood disorder (including leucopenia), as metronidazole may affect blood cell counts during prolonged systemic use. Regular blood monitoring may be required.
• Liver disease: Metronidazole is extensively metabolised in the liver. Patients with severe hepatic impairment may have reduced clearance, leading to drug accumulation. Dose adjustments may be necessary.
• Neurological effects: Prolonged systemic use of metronidazole has been associated with peripheral neuropathy (numbness, tingling, or pain in extremities) and central nervous system effects (seizures, encephalopathy). Treatment should be discontinued if neurological symptoms develop.
• Avoid prolonged or unnecessary use of metronidazole in any formulation to minimise the risk of antimicrobial resistance and adverse effects.

Pregnancy and Breastfeeding

The safety of metronidazole during pregnancy has not been fully established, and its use requires careful medical assessment of the benefit-risk ratio.

Pregnancy: There is limited experience with topical metronidazole use during pregnancy. Topical metronidazole should only be used during pregnancy if your doctor considers it essential. Oral and intravenous metronidazole crosses the placenta and should generally be avoided during the first trimester unless the clinical situation demands it. The EMA and FDA classify systemic metronidazole as requiring caution in pregnancy; some guidelines permit its use in the second and third trimesters for specific indications (such as trichomoniasis or bacterial vaginosis) when the benefit clearly outweighs the risk.

Breastfeeding: Metronidazole passes into breast milk. For topical use, systemic absorption is very low, but you should consult your doctor before using topical metronidazole while breastfeeding. For systemic formulations, it is generally recommended to either discontinue breastfeeding during treatment or to avoid the drug, depending on the clinical situation. Some guidelines suggest pumping and discarding breast milk for 12–24 hours after a single-dose regimen.

Use in Children

Topical metronidazole formulations for rosacea should not be used in children, as there is insufficient clinical experience in paediatric populations for this indication. Systemic metronidazole is, however, used in children for specific infections (such as amoebiasis, giardiasis, and anaerobic infections) at appropriate weight-based doses under medical supervision, as recommended by the BNF for Children and WHO guidelines.

Driving and Operating Machinery

Topical metronidazole does not affect the ability to drive or use machines. Systemic metronidazole may, in rare cases, cause dizziness, confusion, or visual disturbances. If affected, patients should avoid driving or operating machinery until symptoms resolve.

How Does Metronidazole Interact with Other Drugs?

Quick Answer: Metronidazole interacts significantly with alcohol (disulfiram-like reaction), warfarin (increased bleeding risk), lithium (toxicity risk), and phenytoin. Topical formulations have minimal systemic absorption and therefore fewer interaction concerns, but inform your doctor of all medications you are taking.

Drug interactions are primarily a concern with oral and intravenous metronidazole, as topical formulations result in very low systemic absorption. However, it is still important to inform your healthcare provider about all medications, supplements, and herbal products you are currently taking. The following table summarises the most clinically significant interactions:

Major Interactions

Other Noteworthy Interactions

For topical metronidazole, drug interactions are not expected to be clinically significant because systemic absorption is very low (typically less than 1% of the applied dose). However, patients using topical metronidazole alongside oral anticoagulants (such as warfarin) should be aware that even small amounts of systemic absorption could theoretically potentiate anticoagulant effects, and any unusual bruising or bleeding should be reported to a healthcare provider promptly.

What Is the Correct Dosage of Metronidazole?

Quick Answer: For topical rosacea treatment, apply a thin layer of 0.75% gel or 1% cream twice daily (morning and evening) for 3–4 months. Systemic dosage varies by indication and patient group. Always follow your prescriber’s instructions.

Topical Dosage (Rosacea)

Systemic Dosage (Oral / Intravenous)

Systemic metronidazole dosing varies considerably depending on the specific indication, severity of infection, and patient characteristics. The following are general adult dosing guidelines based on EMA, FDA, BNF, and WHO recommendations:

Children

Topical metronidazole for rosacea should not be used in children, as there is insufficient clinical experience. For systemic infections in children, dosing is weight-based and should be determined by a paediatrician. The BNF for Children provides detailed paediatric dosing. As a general guide, the usual oral dose for anaerobic infections in children is 7.5 mg/kg every 8 hours, with adjustments based on age, weight, and infection severity.

Elderly Patients

No specific dose adjustment is required for topical metronidazole in elderly patients. For systemic use in the elderly, caution is advised due to the higher likelihood of hepatic or renal impairment. Pharmacokinetic changes associated with ageing may lead to reduced clearance. Monitor closely for adverse effects, particularly neurological symptoms, and consider lower doses or shorter treatment courses where clinically appropriate.

Missed Dose

If you forget to apply topical metronidazole at the scheduled time, apply it as soon as you remember—unless it is nearly time for your next application. In that case, skip the missed application and continue with your regular schedule. Do not apply a double layer to compensate for a missed dose. The same principle applies to oral formulations: never take a double dose to make up for a forgotten one.

Overdose

Topical metronidazole overdose is extremely unlikely due to the limited systemic absorption from skin application. If accidentally ingested (for example, by a child), seek immediate medical attention or contact your local poison control centre.

Systemic metronidazole overdose may present with nausea, vomiting, ataxia (unsteady gait), and in severe cases, seizures or peripheral neuropathy. Treatment is supportive, as there is no specific antidote. Haemodialysis may be considered in severe cases, as metronidazole is dialysable.

What Are the Side Effects of Metronidazole?

Quick Answer: Topical side effects include burning, stinging, dryness, redness, and itching. Systemic side effects include nausea, metallic taste, and headache. Serious but rare effects include peripheral neuropathy. Report any unusual symptoms to your doctor.

Like all medicines, metronidazole can cause side effects, although not everyone experiences them. The side effect profile differs between topical and systemic formulations due to the vast difference in systemic exposure.

Topical Metronidazole Side Effects (Gel / Cream)

Systemic Metronidazole Side Effects (Oral / IV)

Most side effects from topical metronidazole are mild and temporary, often resolving as the skin adjusts to treatment. If skin irritation is bothersome, you may reduce the frequency of application to once daily until the irritation subsides, then gradually return to twice-daily use. If severe irritation persists, discontinue use and consult your doctor.

For systemic metronidazole, the most frequently reported side effects—nausea and metallic taste—are generally dose-related and resolve after completing the course. Taking oral metronidazole with food can reduce gastrointestinal side effects. The dark discolouration of urine is a harmless effect of metronidazole metabolites and does not indicate a medical problem.

How Should You Store Metronidazole?

Quick Answer: Store metronidazole at room temperature, protected from cold and freezing. Keep out of reach of children. Do not use after the expiry date.

Proper storage of metronidazole ensures the medication remains effective and safe throughout its shelf life. The following storage guidelines apply to all formulations:

• Temperature: Store at room temperature (below 25°C / 77°F). Do not refrigerate topical formulations, and do not freeze any metronidazole product.
• Light: Protect from direct sunlight. Store in the original packaging.
• Children: Keep all metronidazole formulations out of sight and reach of children.
• Expiry date: Do not use metronidazole after the expiry date stated on the packaging. The expiry date refers to the last day of that month.
• Opened tubes: Once opened, topical gel or cream should be used within the period specified on the packaging (typically 6 months for most brands).
• IV solution: Metronidazole infusion solution should be used immediately after opening. Any unused portion should be discarded according to institutional protocols.

Do not dispose of metronidazole via wastewater or household waste. Return unused medication to your pharmacy for proper environmental disposal. These precautions help protect the environment from pharmaceutical contamination.

What Does Metronidazole Contain?

Quick Answer: The active substance is metronidazole (7.5 mg per gram in 0.75% gel; 10 mg per gram in 1% cream). Inactive ingredients vary by formulation but may include carbomer, propylene glycol, parabens, and purified water.

Active Ingredient

All metronidazole formulations contain metronidazole (chemical name: 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol) as the sole active pharmaceutical ingredient. The concentration varies by formulation:

• 0.75% gel – 7.5 mg metronidazole per gram of gel
• 1% cream – 10 mg metronidazole per gram of cream
• IV solution – 5 mg metronidazole per millilitre of solution

Inactive Ingredients (Excipients)

The inactive ingredients differ between formulations and manufacturers. Below are the typical excipients found in metronidazole 0.75% gel (such as Rozex):

• Carbomer (thickening agent)
• Propylene glycol (E1520) – approximately 3% w/w; may cause skin irritation in sensitive individuals
• Methylparaben (E218) – preservative; may cause allergic reactions (potentially delayed)
• Propylparaben (E216) – preservative; may cause allergic reactions (potentially delayed)
• Disodium edetate (chelating agent)
• Sodium hydroxide (pH adjuster)
• Purified water

Appearance

Metronidazole 0.75% gel is a clear to slightly yellow gel that may develop a slightly brownish tint over time, which does not affect its efficacy. Metronidazole 1% cream is a white to off-white smooth cream. The intravenous infusion solution is a clear, colourless to pale yellow solution.

Packaging

Topical metronidazole gel (such as Rozex) is typically supplied in 30 g or 60 g aluminium tubes. Cream formulations are available in similar tube sizes. The IV solution is supplied in polyethylene bags or glass bottles, typically in 100 ml volumes (500 mg/100 ml). Packaging may vary between manufacturers and countries.