How Long Does Metronidazole Stay in Your System?

Medically reviewed by Alex Yampolsky, PharmDMedically reviewed by Alex Yampolsky, PharmD

Metronidazole is an antibiotic commonly used to treat different types of infections. It can be taken anywhere from once per day (if you take an extended-release formulation) to up to four times per day, or every six hours. Common brand name products include Flagyl and Metrogel.

One dose of metronidazole will stay in your system for around 40 to 56 hours, or approximately two days. After two days, the amount of metronidazole remaining after just one dose will be negligible and no longer effective. This is why it must be taken more often than every other day to keep the concentration in your blood high enough to continue treating the infection.

Uses of Metronidazole

Some common approved indications for metronidazole include:

• Bacterial vaginosis: A vaginal infection caused by overgrowth of bacteria such as Gardnerella vaginalis.

• Clostridioides difficile (C. diff) diarrhea: This results from the severe inflammation that the bacteria of the same name can cause in the gastrointestinal tract.

• Diabetic foot infection: This occurs when a foot injury or ulcer becomes infected. This can occur in people with diabetes, nerve damage, or poor blood circulation, as these issues can prevent them from noticing foot sores until they are already infected.

• Giardiasis: An infection of the intestines transmitted through water that is often caused by consuming contaminated water or food.

• Osteomyelitis: A bone infection that may result from untreated or unhealed skin or soft tissue infections.

• Trichomoniasis: A sexually transmitted infection (STI) caused by the parasite trichomonas vaginalis.

Not all these infections are always caused by bacteria susceptible to or can be killed by metronidazole. When possible, healthcare providers may run tests such as gram stains or culture and sensitivity tests to identify whether metronidazole will work against the bacteria causing your infection.

How Long Does Metronidazole Stay in Your System?

Metronidazole has a half-life of about eight hours. The half-life is the amount of time it takes for the concentration of the drug in your blood to be reduced by half. For a drug to be effectively eliminated from your body or reduced to a negligible amount, it takes five to seven half-lives, so for metronidazole, that is 40 to 56 hours or approximately two days.

Half-lives of drugs are used to help determine dosing schedules. Metronidazole is typically dosed between two and four times per day, or around every 6 to 12 hours. Dosing depends on what type of bacteria is causing the infection and where the infection is in the body. Certain bacteria or locations in the body may require higher or lower concentrations of metronidazole for effective treatment, meaning you need to keep taking the drug fairly often to remain at or above that blood concentration.

Side Effects of Metronidazole

More common side effects include:

• Nausea/vomiting

• Diarrhea

• Abdominal pain or cramps

• Metallic taste in the mouth

• Headache

• Dizziness

• Loss of appetite

Less common side effects include:

• Dry mouth

• Dark or reddish-brown urine

• Fatigue

• Mouth sores or tongue swelling

What Precautions Should I Consider for Metronidazole?

Be sure your healthcare provider is aware of your full medical history to provide the most optimal antibiotic and dose to treat your infection. They should be aware if you:

• Have liver disease or kidney disease, as the dose of metronidazole may need to be adjusted to prevent a buildup of medication in your body, which can increase the risk of side effects.

• Have a blood disorder, such as anemia (low red blood cells), leukopenia (low white blood cells), or thrombocytopenia (low platelets), as metronidazole has the potential to worsen these conditions.

• Have a nervous system disorder such as epilepsy, migraines, or multiple sclerosis, as metronidazole has the potential to cause or worsen seizures, dizziness, headaches, and confusion.

Interactions

There are a few important drug interactions you should be aware of when you start taking metronidazole:

• Warfarin and other blood thinners. Metronidazole may increase the blood-thinning effect of warfarin by slowing down your body’s ability to metabolize or clear it. This may lead to higher levels of warfarin in your body and an increased risk of bleeding. Watch for signs of bleeding, such as unusual bruising, blood in your urine or stool, or bleeding gums while brushing your teeth.

• Lithium. Metronidazole can affect your kidneys’ ability to clear lithium from your body, and increased levels can build up, leading to an increased risk of side effects. Signs of lithium toxicity may include nausea, vomiting, diarrhea, tremors, muscle weakness, and seizures.

• Phenytoin and phenobarbital. The levels of these drugs can be increased by metronidazole, and the reverse is also true: they can decrease levels of metronidazole. Dose adjustments may be necessary to avoid increased side effects from these antiseizure medications or lowered effectiveness from the antibiotic.

Be sure all of your healthcare providers and your pharmacist are aware of your full medication list so they can best advise you on how to avoid drug interactions or make necessary dose adjustments.

Is There an Interaction Between Metronidazole and Alcohol?

Data on the presence of disulfiram-like reactions with metronidazole is conflicting and not definitive, but the possibility of a reaction is included in the manufacturer's labeling, so it is best to avoid alcohol. Consuming alcohol while taking metronidazole can lead to a severe reaction that causes nausea, vomiting, flushing, headache, and increased heart rate.

Wait at least three days after you take your last prescribed dose of metronidazole before drinking alcohol to make sure that the antibiotic is sufficiently cleared from your body and will not cause this disulfiram-like reaction.

Summary

Metronidazole is an important antibiotic used to treat many different types of infections. It’s best to wait until at least three days after your last dose of metronidazole before consuming alcohol, as this will give your body the time it needs to clear the drug almost completely from your body.

As with any antibiotic, complete your entire course as prescribed, even if you feel better before it’s finished. Stopping the medication before your course ends can cause the infection to return and contribute to antibiotic resistance.

Read the original article on Verywell Health.