Metronidazole

Metronidazole is prescribed to treat specific infections caused by parasites or anaerobic bacteria (bacteria that grow in low-oxygen environments). It is approved for:

• Trichomoniasis — a sexually transmitted infection caused by a parasite; both partners should be treated
• Amebiasis — a parasitic infection that can cause severe intestinal illness (amebic dysentery) or a liver abscess
• Anaerobic bacterial infections — serious infections affecting areas such as:
  • Abdomen (including peritonitis and abscesses)
  • Skin and soft tissue
  • Gynecological organs
  • Bones and joints
  • Lower respiratory tract (lungs)
  • The nervous system (brain and spinal area)
  • The lining of the heart (endocarditis)
  • Blood (septicemia)

Metronidazole works by entering bacteria or parasites and getting converted into a toxic, reactive form inside the organism. This reactive form attacks the organism's DNA — disrupting its structure and blocking it from being copied. Without functioning DNA, the bacteria or parasite can't survive or reproduce. This process works best in low-oxygen (anaerobic) environments, which is why metronidazole is uniquely suited to fighting anaerobic bacteria and certain parasites.

Once activated, metronidazole creates a concentration gradient that actually pulls more of the drug into the organism — making it even more effective at delivering a lethal dose of DNA damage to the target.

Metronidazole is taken by mouth as a tablet or liquid. The dosing schedule varies by infection type — some infections are treated with a single large dose on one day, while others require doses taken two or three times daily for several days up to ten days. Serious anaerobic bacterial infections are often started with intravenous (IV) metronidazole in a hospital setting before switching to oral treatment.

• Can be taken with or without food — food may slightly delay absorption but doesn't meaningfully reduce effectiveness
• Complete the entire prescribed course, even if symptoms improve early
• If you miss a dose, follow your prescriber's or pharmacist's guidance — do not double up

Stomach-related side effects are the most common complaint with metronidazole. Here's what to expect:

• Nausea (most frequent)
• Metallic or unpleasant taste in the mouth
• Headache
• Loss of appetite
• Vomiting or diarrhea
• Stomach cramping or upset
• Constipation
• Furry tongue or mouth sores (sometimes linked to a yeast overgrowth)

More seriously, metronidazole can affect the nervous system — causing seizures, confusion, numbness or tingling in the limbs, or unsteady walking. Severe skin reactions (blistering, peeling) and a drop in white blood cells have also been reported. Get medical help right away if any of these occur.

• Cancer risk in animals: Metronidazole caused tumors in mice and rats in studies. Its cancer risk in humans is not established, but it should only be used for confirmed, appropriate infections — not as a precaution or for minor illnesses.
• Nervous system effects: Seizures, brain dysfunction (encephalopathy), nerve damage (peripheral neuropathy), and aseptic meningitis (inflammation around the brain) have all been reported. These are generally reversible after stopping the drug, but any neurological symptoms warrant prompt medical attention.
• Severe skin reactions: Rare but life-threatening skin conditions like Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported. Stop the medication immediately if you develop a severe rash, blistering, or skin peeling.
• Cockayne syndrome: This medication is contraindicated in people with this rare genetic condition due to risk of rapid, fatal liver failure.
• Blood cell changes: May cause a mild, temporary decrease in white blood cells (leukopenia); use with caution in patients with blood disorders.
• QT prolongation: May affect heart electrical activity, especially when combined with other drugs that do the same — tell your doctor all medications you take.

Metronidazole has several important interactions to know about before starting treatment:

• Alcohol and propylene glycol — causes a severe reaction (nausea, vomiting, flushing, headaches); avoid completely during treatment and for at least 3 days after your last dose
• Disulfiram (Antabuse) — can trigger serious psychiatric reactions; do not combine, and wait at least 2 weeks after stopping disulfiram before taking metronidazole
• Warfarin and coumarin blood thinners — metronidazole increases their blood-thinning effect, raising bleeding risk; INR monitoring is required
• Lithium — metronidazole can raise lithium levels to toxic levels; blood monitoring is needed
• Busulfan (chemotherapy) — metronidazole raises busulfan levels, increasing risk of serious toxicity; avoid combining unless no alternative exists
• Phenytoin or phenobarbital (seizure medications) — may speed up metronidazole's removal from the body, reducing its effectiveness; may also impair clearance of phenytoin
• Cimetidine (stomach acid reducer) — slows metronidazole's breakdown, potentially raising its blood levels
• QT-prolonging drugs — combining may increase the risk of abnormal heart rhythms

This is not a complete list of interactions. Always talk with your doctor or pharmacist before starting, stopping, or changing any medication.

• Tell your doctor if you are allergic to metronidazole or any other nitroimidazole-class drug
• Do not take if you have Cockayne syndrome — risk of rapid, severe liver failure
• Do not take if you have used disulfiram in the past 2 weeks
• Tell your doctor if you are in your first trimester of pregnancy — metronidazole is contraindicated for trichomoniasis treatment during this period
• If you are breastfeeding, discuss with your doctor — you may need to pump and discard breast milk during treatment and for 48 hours after the last dose
• Tell your doctor if you have liver disease — the dose may need to be reduced, especially with severe liver impairment
• Tell your doctor if you have a history of seizures or nerve problems
• Tell your doctor if you have a history of blood disorders
• If you are on hemodialysis (kidney dialysis), your doctor may need to adjust your dosing schedule

Taking too much metronidazole can cause nausea, vomiting, and ataxia (loss of coordination and balance). Higher overdose levels have been linked to seizures and nerve damage. There is no specific antidote — treatment focuses on managing symptoms and supporting the patient. Metronidazole can be removed from the body through dialysis if needed.

Metronidazole works selectively against organisms that live in low-oxygen environments. Once inside a susceptible bacterium or parasite, it is converted into a reactive form that directly damages DNA, preventing the organism from reproducing and ultimately killing it. Both the original drug and one of its breakdown products (the hydroxyl metabolite) have this antibacterial activity. It does not work against aerobic bacteria (those that require oxygen), so mixed infections may require additional antibiotics.

Metronidazole is well absorbed after oral dosing, with blood levels peaking roughly within one to two hours on an empty stomach (food delays the peak but doesn't reduce the total amount absorbed). It distributes widely throughout the body — reaching the cerebrospinal fluid, saliva, breast milk, and even pus in liver abscesses. Less than 20% is bound to proteins in the blood, meaning most of the drug is free and active. The body processes it mainly in the liver and clears it primarily through the urine (60–80%), with the remainder in stool. The average half-life — the time it takes the body to clear half the dose — is about 8 hours in healthy adults.

Store at 20° to 25°C (68° to 77°F) . Protect from light. Dispense in a tight, light-resistant container as defined in the USP, with a child-resistant closure (as required).