If you accidentally took a double dose of metronidazole (Flagyl), one extra dose is unlikely to harm you, according to the NHS. Nausea and other stomach upset, sometimes with a headache, are the most common side effects on the label. Call your pharmacist or Poison Control at 1-800-222-1222 before you take your next dose.
The FDA label for metronidazole tablets also carries a nerve warning. It says patients should be told to "stop the drug and report immediately to their physicians if any neurologic symptoms occur." Those include numbness or tingling in your hands or feet, trouble walking steadily, slurred speech, and confusion.
This article provides general information about medication management and is not a substitute for professional medical advice. Always consult your doctor or pharmacist before making changes to your medication schedule. Consult your doctor or pharmacist for advice specific to your medications.
What Happens If You Take 2 Metronidazole at Once?
Your blood level goes higher than usual for a while, then falls. The label says levels rise in proportion to the dose and peak 1 to 2 hours after a dose: about 12 mcg/mL after 500 mg and 40 mcg/mL after 2,000 mg. The average half-life in healthy people is eight hours.
What two pills add up to depends on your strength (from the tablet label and the Flagyl capsule label):
- Two 250 mg tablets: 500 mg
- Two 375 mg Flagyl capsules: 750 mg
- Two 500 mg tablets: 1,000 mg (1 gram)
Doctors sometimes prescribe more at once on purpose: CDC's treatment guidelines list a single 2-gram dose for trichomoniasis in men. That is a dose picked for one infection, not a safe limit for you.
The clearest look at 2 grams at once comes from a 2018 trial in The Lancet Infectious Diseases. Of 623 women with trichomoniasis, 311 were assigned 2 grams at once (four 500 mg pills) and 312 got 500 mg twice a day for 7 days. Among the 540 seen at follow-up, side effects were similar in both groups. Nausea was the most common (23%), then headache (7%) and vomiting (4%). That was a planned dose taken at a clinic, not an extra pill on top of a course.
Can I Take 2 Metronidazole 500 mg at Once for BV?
Only if your prescription tells you to. For bacterial vaginosis (BV), the metronidazole pill regimen in CDC's BV guidelines is 500 mg twice a day for 7 days. CDC's single 2-gram metronidazole dose is for trichomoniasis in men.
More at once is not always better. In the same Lancet trial, women with trichomoniasis who took the one-time 2-gram dose were more likely to still test positive 4 weeks after treatment than women on the 7-day course (19% vs 11%). That trial was about trichomoniasis, not BV. If you doubled up to make up for a missed dose, see the missed antibiotic dose guide.
Can You Overdose on Metronidazole?
Yes. The label's overdose section says single doses of up to 15 grams have been reported in suicide attempts and accidental overdoses. The symptoms reported were nausea, vomiting, and ataxia, which means losing your balance and coordination. Fifteen grams is 30 tablets of 500 mg. That is the top of what was reported, not a safe line.
The label's overdose section also lists nerve damage, from a different setting: in studies that used metronidazole alongside radiation to treat cancer, seizures and peripheral neuropathy (nerve damage in the hands and feet) were reported after 5 to 7 days of 6 to 10.4 grams every other day.
There is no specific antidote. The label says treatment should be "symptomatic and supportive," meaning doctors treat the symptoms.
A 2022 case report near the top of search results describes an 18-year-old man who developed ventricular fibrillation, a dangerous heart rhythm, after a metronidazole overdose. He first said he swallowed about 30 tablets of 200 mg in a suicide attempt; his account later changed. The authors call it, to their knowledge, the first reported case of ventricular fibrillation from metronidazole overdose alone.
Metronidazole Overdose Symptoms That Mean Stop and Call
Stomach upset is the usual reaction. Nerve symptoms are rare, but they are the ones the label says to act on right away.
| What you notice | What the label calls it | What to do |
|---|---|---|
| Nausea, upset stomach, headache, or vomiting once | The most common side effects | Call your pharmacist if it does not settle |
| Numbness, tingling, burning, or pain in your hands or feet | Peripheral neuropathy | Stop the drug and report it to your doctor right away |
| Unsteady walking, clumsiness, slurred speech, dizziness, or confusion | Encephalopathy, with balance and speech problems | Stop the drug and report it to your doctor right away |
| Any rash, blisters, or a rash with fever | A possible serious skin reaction | Stop the drug and get medical care right away |
| A seizure, collapse, trouble breathing, or the person can't be woken up | An emergency | Call 911 |
The actions in rows 2 to 4 come from the FDA label. The 911 row is from MedlinePlus, which lists nausea, vomiting, loss of muscle coordination, seizures, and numbness or tingling in the hands or feet as possible overdose symptoms.
In a study of adults 66 and older in Ontario, about 0.25% of people prescribed metronidazole went to the hospital with a nerve problem within 100 days. The two kinds behave differently.
Numbness and tingling in the hands and feet has mostly followed long use. A 2018 review found that 31 of 40 published cases involved more than 42 grams in total (more than 4 weeks of treatment).
Balance, speech, and thinking problems are harder to predict. A 2011 review of 64 cases found they did not seem to depend on dose or treatment length, and 11% of those patients had taken metronidazole for less than 72 hours. The label says these symptoms are generally reversible within days to weeks after stopping. So the signs above matter even early in a short course.
How Long Do I Have to Wait to Take Another Metronidazole?
Ask before you take it. The label does not say what to do about your next dose after a double dose. Schedules on the label range from a single dose to every 6 hours, so the answer depends on your prescription.
Have these ready when you call your pharmacist or Poison Control:
- The strength on your bottle and how many you took.
- When you took each one. If your second pill was only early, see how early you can take an antibiotic.
- Your schedule, such as three times a day or twice a day, and what it treats.
- Pregnancy or breastfeeding, liver or kidney disease, and your other medicines.
Call now rather than waiting if:
- You are pregnant. In its trichomoniasis dosing, the label says the one-day course should not be used for pregnant patients because it "results in higher serum levels which can reach the fetal circulation."
- You take warfarin or lithium. The label says metronidazole can strengthen warfarin's effect and has raised lithium levels in people on relatively high lithium doses.
- You have liver disease, kidney failure, or are an older adult. The label says liver problems slow how fast metronidazole is cleared, and it recommends watching older patients and people with end-stage kidney disease for side effects.
- You took more than one extra dose. The NHS tells people to get urgent advice if they take more than 1 extra dose and have any new symptoms.
Unless one of the stop signs above applies, do not skip doses or stop the course on your own to "even it out." Ask first. The label warns that skipping doses may make treatment less effective and make resistance more likely.
Is a Metallic Taste or Dark Urine a Sign of Too Much Metronidazole?
Usually not. Both can happen at normal doses.
The label says "a sharp, unpleasant metallic taste is not unusual."
Dark urine is rare. The label reports it in about one patient in 100,000 and says it is almost certainly a breakdown product of the drug that "seems to have no clinical significance."
Other things can look like a reaction to the extra pill:
- Cramps, flushing, or vomiting after a drink. The label warns these may occur with alcohol during or after a course. See metronidazole and alcohol.
- Vaginal itching or discharge. The label says Candida (yeast) infections can show up or get worse during treatment.
Accidentally Gave Your Dog a Double Dose of Metronidazole?
Call your veterinarian now. The numbers on this page are for people, not pets.
Pet Poison Helpline says antibiotic overdose in pets can cause anything from stomach upset to seizures and death, and treatment depends on the drug, the amount, and how long ago it was swallowed. It is open 24/7 at (855) 764-7661 and charges a one-time consultation fee of $89 per incident. The ASPCA Animal Poison Control Center is at (888) 426-4435, and a consultation fee may apply.
Have the tablet strength, your dog's weight, and the time of each dose ready. For day-to-day routines, see managing your pet's medications.
How Pillo Helps
A 7-day course at two or three doses a day, like the CDC BV regimen or the label's trichomoniasis course, is 14 to 21 doses. That is a lot of chances for a "did I already take that?" moment. Pillo's persistent alarm keeps going until you respond, so your dose history answers the question instead of your memory.
If your dog takes metronidazole too, Dependents Management (part of the paid plan) gives your pet its own schedule, separate from yours.
For other drugs, see took your medication twice, or a double dose of amoxicillin or doxycycline.
Frequently Asked Questions
What happens if I accidentally took two metronidazole?
Nausea, stomach upset, or a headache are the most common side effects on the label. The NHS says an extra dose is unlikely to harm you. Call your pharmacist or Poison Control at 1-800-222-1222 before your next dose. If you notice numbness, tingling, or trouble with balance, the FDA label says to stop the drug and report it to your doctor right away.
Is a double first dose of metronidazole normal?
Only if your prescription says so. The label allows its one-day 2-gram trichomoniasis course as one dose or as two 1-gram doses on the same day. If your bottle says one tablet per dose, a doubled first dose was a mistake. Ask your pharmacist about the next one.
Should I skip my next dose after a double dose of metronidazole?
Do not decide on your own. Metronidazole has an average half-life of eight hours. Your pharmacist or Poison Control will tell you whether to take the next one on time. The exception is nerve symptoms or a rash: the label says to stop the drug and get medical advice right away.
I accidentally took my dog's metronidazole. What should I do?
Read the strength on your pet's label, then call Poison Control at 1-800-222-1222 with that number and how many you took. If you also take metronidazole yourself, mention that prescription too.
Sources
- FDA label on DailyMed: Metronidazole tablets USP, 250 mg and 500 mg (Teva)
- FDA label on DailyMed: Flagyl (metronidazole) capsules, 375 mg (Pfizer)
- Kissinger et al., Single-dose versus 7-day-dose metronidazole for the treatment of trichomoniasis in women: an open-label, randomised controlled trial (Lancet Infect Dis, 2018)
- Kuriyama et al., Metronidazole-induced central nervous system toxicity: a systematic review (Clin Neuropharmacol, 2011)
- Goolsby et al., Clinical relevance of metronidazole and peripheral neuropathy: a systematic review of the literature (Int J Antimicrob Agents, 2018)
- Daneman et al., Metronidazole-associated Neurologic Events: A Nested Case-control Study (Clin Infect Dis, 2021)
- Elgassim et al., A Rare Case of Metronidazole Overdose Causing Ventricular Fibrillation (Cureus, 2022)
- CDC: Trichomoniasis, STI Treatment Guidelines, 2021
- CDC: Bacterial Vaginosis, STI Treatment Guidelines, 2021
- MedlinePlus: Metronidazole
- NHS: How and when to take or use metronidazole
- Pet Poison Helpline: Antibiotic Overdose in Pets
- Pet Poison Helpline: About
- ASPCA Animal Poison Control Center
