Oral Flagyl Explained: Usage, Doses, And Side Effects

Flagyl is the brand name for metronidazole, an antibiotic used to treat certain bacterial and parasitic infections (via the Cleveland Clinic). It is not indicated for viral infections (e.g., the common cold, flu). Flagyl is associated with a variety of adverse effects such as confusion, headache, nausea, vomiting, diarrhea, metallic taste, tingling in the hands or feet, and a change in vaginal discharge. It is contraindicated with alcohol use, and may interact with a number of other medications such as the antacid Tagamet and birth control pills. 

Flagyl is most commonly taken by mouth in pill form, though it may also be administered intravenously, vaginally, or rectally (via a 2019 review in the European Review for Medical and Pharmacological Sciences). Oral Flagyl was initially used to treat trichomoniasis, a sexually transmitted disease caused by the parasite Trichomonas vaginalis. It has subsequently been used to treat other parasitic infections such as giardiasis and amebiasis, both of which occur in the intestine and can cause severe diarrhea. Flagyl is also effective against the bacteria Helicobacter pylori and Clostridium difficile, which are associated with ulcers and diarrhea, respectively. It's used to treat bacterial vaginosis, Crohn's disease, and rosacea as well. In addition to treating infections, Flagyl can be used preventatively to reduce the risk of infections occurring after surgery.

Here's what you need to know about oral Flagyl, including its usage, doses, and side effects.

Flagyl has been the first line treatment for anaerobic bacterial infections for more than fifty years (via a 2018 study published in Therapeutic Advances in Infectious Disease). Anaerobic (as opposed to aerobic) bacteria thrive and proliferate in the absence of oxygen. For example, these bacteria are normal inhabitants of the mouth and intestine. Along with aerobic bacteria, they are often culprits behind infections in these organs. Consensus guidelines recommend treating the anaerobic bacteria even in cases where the anaerobic bacterium has not yet been identified. Oral (as well as IV) Flagyl is the medication frequently used to treat these infections, as it is effective and well tolerated. 

Among the species of anaerobic bacteria that are vulnerable to treatment with Flagyl are Clostridium, Bacteroides, Fusobacterium, and Peptostreptococcus. (via DailyMed). Sites of infection with these bacteria (other than the mouth and intestine) include the skin, liver (abscess), abdomen (peritonitis), bone and joints, central nervous system (meningitis, brain abscess), lower respiratory tract (pneumonia, lung abscess), and the heart (endocarditis). Gynecologic infections (endometritis, tube-ovarian abscess) and septicemia (blood poisoning) are also caused by anaerobic bacteria. 

Only infections that are proven or very likely to be caused by these bacteria should be treated with Flagyl. Overprescribing of any antibiotic can lead to increased antibiotic resistance. Antibiotic susceptibility testing for Flagyl is advised when available. This test determines whether Flagyl is effective against a specific bacterium. 

Crohn's disease is one of the two major inflammatory bowel diseases (IBD) that cause chronic inflammation in any part of the gastrointestinal tract (via a 2016 review in the World Journal of Gastroenterology). The prevalence of Crohn's disease in the U.S. is 200 out of 100,000 people and increasing. 

The population of microorganisms in the intestine, collectively termed the gut microbiota, plays a major role in the inflammatory process in Crohn's disease. People with Crohn's disease often have a reduced diversity of gut bacteria. In genetically susceptible individuals, certain disease-causing bacteria (e.g., Bacteroides fragilis) trigger an abnormal immune response that results in chronic gut inflammation. In contrast, other species of intestinal bacteria (like Lactobacillus) have a protective effect against the development of Crohn's disease and other inflammatory bowel diseases.

The use of Flagyl in the treatment of active Crohn's disease has been shown to decrease disease activity (though modestly), and was found to be particularly effective for disease confined to the colon. Antibiotics such as Flagyl may exert their beneficial effects by changing the population of gut bacteria. Specifically, they encourage the growth of protective bacteria while also killing harmful bacteria such as Bacteroides fragilis. When administered at low doses for three months and tolerated well, Flagyl was also shown to significantly reduce the rate of recurrence of Crohn's disease one year after surgery (via a 2019 study in the Journal of Crohn's and Colitis). 

Periodontitis is a serious infection of the gums that causes inflammation and damage to the tissues and bone supporting the teeth (via a 2022 research article published in Frontiers in Oral Health). The development and progression of periodontitis is primarily caused by infection with the bacterium Porphyromonas gingivalis. This pathogen contributes to periodontal biofilms (aggregates of bacteria and other microbes) and, owing to its ability to evade immune defenses, is very persistent. P. gingivalis is associated with progressive bone loss, and may be resistant to conventional treatment via scaling and root planning. 

In cases of periodontitis that do not respond to conventional therapy, treatment with Flagyl is an effective alternative, as it eliminates P. gingivalis. In addition to directly killing the bacteria, Flagyl also works by modulating the immune system to reinforce the killing of P. gingivalis by specific immune cells called polymorphonuclear neutrophils (PMN). Flagyl can also be used as a treatment for gingivitis, periodontal disease, and halitosis, as well as a prophylaxis for dental surgery.

Some common adverse effects of Flagyl are generally not severe enough to require medical attention, notes Healthline. These include nausea, vomiting, diarrhea, constipation, heartburn, abdominal pain, poor appetite, and metallic taste in the mouth. Women may experience yeast infections and vaginal discharge. Though uncommon, several serious side effects affecting the brain and nervous system can occur from taking Flagyl. Some have reported the development of encephalopathy (brain malfunction), which can lead to problems with simple movements such as walking as well as dizziness and impaired speech. Other brain-related effects include seizures, meningitis marked by fever and severe headache, and nerve damage that can cause pain and numbness in the hands or feet. 

Another serious effect of Flagyl is neutropenia, a deficiency of infection-fighting immune cells known as neutrophils. Neutropenia can increase vulnerability to new infections. Flagyl has also been linked to a serious skin condition called Stevens-Johnson syndrome. This rare condition can cause painful blisters on the skin, as well as fever and fatigue. These symptoms require emergency medical attention. Any severed allergic reaction to Flagyl (e.g., shortness of breath, tongue or throat swelling) is also a medical emergency. 

Oral Flagyl also has a black box warning from the FDA that pertains to the risk of cancer. Some animal studies testing Flagyl resulted in the development of cancer, though the animals were given high doses of Flagyl for an extended period. Based on these studies, Flagyl may be carcinogenic (cancer-causing) in humans. 

According to Medscape, well-controlled studies to determine whether Flagyl is safe to use during pregnancy are limited. Most human studies investigating exposure to Flagyl during pregnancy have not shown an increased risk of birth defects or infant cancer. 

Though unconfirmed, one study reported an elevated risk of cleft lip in newborns of women who took Flagyl during pregnancy. While it is known that Flagyl is transferred across the placenta and reaches the fetus, evidence from animal studies testing the highest recommended dose shows no harm to the fetus. However, there is no knowledge of whether it has any impact on the development of the human fetus. Ultimately, the physician should weigh the potential risks against the potential benefits when prescribing Flagyl during pregnancy. 

Flagyl passes into human milk at concentrations closely matching levels found in the blood. Due to the potential increased risk of cancer based on animal studies, women should discontinue breastfeeding if they must stay on the medication. Otherwise, mothers can pump and discard breast milk when taking Flagyl, as well as within 24 hours after stopping the medication. Stored breast milk or formula can then be fed to the baby.