Metronidazole can cause your urine to turn dark, and the change is harmless. The color shift is a recognized side effect listed in the drug’s prescribing information, caused by metabolites of the medication being excreted through your kidneys. The shade people report ranges from reddish-brown to what clinicians have described as “cola-colored,” and it clears up once you stop taking the drug. Despite how alarming it can look, the discoloration itself does not signal kidney damage or any other organ problem.
What the Color Change Actually Looks Like
People taking metronidazole don’t always describe their urine the same way, which can add to the confusion. Published case reports have used terms like “cola-colored” and “reddish discolouration” to describe the change.1PubMed Central. Urine Discoloration Associated With Metronidazole: A Rare Occurrence2PubMed Central. Urine Discoloration Associated with Metronidazole: A Case Report In practice, the range runs from a dark amber or brownish tint all the way to something that looks frankly reddish. The exact shade depends on how concentrated your urine is, how much water you’ve been drinking, and your individual metabolism. If you’re well-hydrated, the color may look like dark tea. If you’re dehydrated, it can appear closer to brown or deep red-brown.
The color change tends to be most noticeable in a toilet bowl under fluorescent bathroom lighting, which can make even mildly darkened urine look more dramatic than it is. Some people never notice it at all, particularly if they already have darker urine from not drinking enough fluids. Others spot it immediately and understandably worry something is wrong.
Why Metronidazole Changes Urine Color
Your body breaks metronidazole down in the liver into several metabolites, including hydroxy-metronidazole and an acid metabolite. These breakdown products are water-soluble pigments. As your kidneys filter them out of your bloodstream and concentrate them in urine, the pigments tint the liquid dark. The drug itself is colorless in solution, so the color you see is not the pill dissolving through your system. It’s the chemical byproducts your liver creates while processing it.
This is essentially the same reason beets turn some people’s urine pink or B vitamins turn it neon yellow. A compound enters the body, gets metabolized, and the resulting waste products happen to have a color. With metronidazole, the metabolites absorb light in a way that produces brownish-red hues. The pigments are chemically inert by the time they reach the urine, meaning they don’t interact with the bladder lining or kidneys in any damaging way.
How Quickly It Starts and Stops
The onset varies from person to person, but the color change can show up within a day or two of starting the medication. In one documented case involving a patient treated for a C. difficile infection, the drug was accidentally stopped for about 34 hours. During that gap the urine returned to its normal color, and it darkened again when metronidazole was restarted.1PubMed Central. Urine Discoloration Associated With Metronidazole: A Rare Occurrence In another case, a four-year-old boy’s urine color reverted to normal within 24 hours of discontinuing the drug.2PubMed Central. Urine Discoloration Associated with Metronidazole: A Case Report
The takeaway is that this is tightly linked to active drug metabolism. Once metronidazole clears your system, which happens relatively quickly given its half-life of around eight hours, the pigmented metabolites wash out within a day or so. You don’t need to do anything special to speed this along. Drinking more water will dilute your urine and may make the color less dramatic while you’re still on the medication, but the discoloration will resolve on its own once the course is finished.
Is It Actually Rare?
Drug references list urine discoloration as a “known” side effect, yet the published medical literature describes it as rarely reported.2PubMed Central. Urine Discoloration Associated with Metronidazole: A Case Report This discrepancy probably says more about reporting patterns than about how often it actually happens. A patient who notices dark urine, mentions it to a pharmacist, and gets reassured that it’s normal will never generate a case report. The cases that do get published tend to involve unusual circumstances: a hospitalized patient whose urine output is being closely monitored, a child whose parents are alarmed, or a situation where the dark urine initially prompted an expensive diagnostic workup before someone realized the drug was the cause.
Informal surveys and pharmacist experience suggest the color change is more common than the published case count implies. Many people simply never look closely at their urine, or they attribute the change to dehydration or something they ate. The effect also varies with dose. Higher doses, such as those used for C. difficile or severe anaerobic infections, are more likely to produce enough pigmented metabolite to visibly tint the urine than the lower doses sometimes used for other conditions.
When Dark Urine on Metronidazole Is Not Just the Drug
The fact that metronidazole can cause harmless dark urine does not mean every instance of dark urine while taking it is harmless. There are situations where a color change signals a different problem, and being on metronidazole can make it easy to wave away something that deserves attention.
The conditions that also produce dark urine and could overlap with a metronidazole course include:
- Dehydration: Highly concentrated urine is dark amber to brownish, and infections that cause fever, vomiting, or diarrhea (the very conditions metronidazole treats) can dehydrate you. The fix is straightforward: increase fluid intake and see if the color lightens.
- Blood in the urine: Hematuria can produce pink, red, or brown urine. If you have pain with urination, flank pain, or visible clots, the color is not from metronidazole.
- Liver problems: Dark urine combined with yellowing of the eyes or skin, pale stools, and upper abdominal pain points toward a liver or biliary issue. A very early case report from 1965 documented jaundice with dark urine in a patient taking metronidazole, though this was an exceedingly unusual presentation.3JAMA. Jaundice Associated With Metronidazole Liver injury from metronidazole is extremely rare, but if you develop yellow eyes or skin alongside dark urine, that warrants prompt medical evaluation regardless of what medication you’re on.
- Rhabdomyolysis: Severe muscle breakdown releases myoglobin, which turns urine dark brown. This is a medical emergency and usually accompanied by muscle pain and weakness.
The simplest way to distinguish harmless drug-related color change from something worrying is context. If the dark urine appeared shortly after starting metronidazole, you feel fine otherwise, and there’s no pain, fever, or jaundice, the drug is the most likely explanation. If you have any accompanying symptoms beyond the color change, it’s worth calling your doctor rather than assuming the medication is responsible.
Does It Affect Urine Tests or Dipstick Results?
This is a practical concern that doesn’t get enough attention. If you’re undergoing urinalysis while on metronidazole, the dark pigment can potentially interfere with visual readings on urine dipsticks. Dipstick pads work by color change, and a background of dark brown urine can make it harder to read results accurately, particularly for blood and bilirubin. Automated analyzers are less susceptible to this than manual visual reads, but it’s still worth telling the lab or your doctor that you’re taking metronidazole if you need a urine test during treatment.
The interference is cosmetic in a sense: metronidazole metabolites don’t actually contain blood or bilirubin, but they can cause a false visual impression of their presence on a dipstick. A microscopic analysis of the urine sample will clarify the situation, since the metabolites don’t produce actual red blood cells or bilirubin crystals under a microscope. The key practical step is simply disclosure. Mention the medication to whoever is ordering or interpreting the test.
Who Is More Likely to Notice the Change
The color effect has been documented in both children and adults. The published pediatric case involved a four-year-old boy whose parents noticed reddish discoloration during treatment with oral metronidazole.2PubMed Central. Urine Discoloration Associated with Metronidazole: A Case Report An adult case involved a patient hospitalized for C. difficile infection who developed cola-colored urine and suffered no harmful effects from the discoloration.1PubMed Central. Urine Discoloration Associated With Metronidazole: A Rare Occurrence Neither age group seems immune to the effect, and there’s no strong evidence that it’s systematically more common in one population than another.
That said, people with kidney impairment may handle the drug’s metabolites differently. If your kidneys don’t clear waste as efficiently, metabolites may accumulate to higher concentrations, potentially making the color change more pronounced. People with liver disease may also metabolize the drug differently, though in their case any change in urine color warrants closer scrutiny because of the overlapping concern about biliary dysfunction.
Parents of young children tend to notice the change more often simply because they’re more likely to see the toilet bowl before it’s flushed. In hospital settings, nurses monitoring urine output may flag the color change, which is how several of the published case reports originated. Outpatients taking a short course for something like a dental infection or bacterial vaginosis may never notice at all, especially if they finish the prescription in a week and aren’t paying close attention.
Should You Stop Taking the Medication?
No. Urine discoloration alone is not a reason to discontinue metronidazole. Both published case reports emphasize that the patients experienced no harmful effects from the color change itself.1PubMed Central. Urine Discoloration Associated With Metronidazole: A Rare Occurrence Stopping an antibiotic early because of a cosmetic side effect risks leaving the underlying infection incompletely treated, which can lead to relapse or resistance. If your doctor prescribed metronidazole for an anaerobic infection, C. difficile, or another indication, the benefit of completing the course far outweighs the inconvenience of odd-looking urine for a few days.
The only scenario where dark urine should prompt you to contact your prescriber is if it’s accompanied by other symptoms: pain, nausea, jaundice, extreme fatigue, or blood that you can clearly distinguish from the brownish tint. Those symptoms may point to a different problem that happens to coincide with your antibiotic course, and they deserve evaluation on their own merits.
Other Medications That Change Urine Color
Metronidazole is far from the only drug that turns urine an unexpected color. Knowing this broader context can be reassuring if you’ve been startled by what you saw in the toilet.
- Nitrofurantoin: Another antibiotic commonly used for urinary tract infections. It can turn urine rust-colored or dark yellow-brown.
- Rifampin: Used for tuberculosis and some other infections. It turns urine, tears, and sweat bright orange-red, sometimes staining contact lenses.
- Phenazopyridine: A bladder pain reliever often taken alongside antibiotics for UTIs. It turns urine vivid orange.
- Chloroquine: An antimalarial that can cause brown discoloration of urine.
- Certain laxatives: Senna-based laxatives can produce reddish-brown urine.
In all of these cases, the mechanism is similar: the drug or its metabolites happen to be pigmented, and they’re concentrated in urine as the kidneys do their normal filtering job. None of these color changes are harmful on their own. The pattern across medications reinforces the point that your kidneys are working exactly as designed. They’re clearing waste products from the blood, and some of those waste products have color.
What Pharmacists Wish People Knew
Pharmacists and prescribers sometimes mention this side effect when dispensing metronidazole, but it’s easy to miss in the flood of information that comes with a new prescription. The ideal moment to learn about it is before you take the first dose, not when you’re staring at the toilet wondering if something has gone terribly wrong. If you’re picking up a metronidazole prescription, it’s worth asking your pharmacist about expected side effects, including the urine change. The other common ones you’ll hear about are a metallic taste in the mouth and the strong recommendation to avoid alcohol during treatment and for at least 48 hours afterward.
If you’ve already been surprised by the color and are reading this after the fact, the reassurance is straightforward: this is your body doing exactly what it should with the drug, and the color will go away. You don’t need additional tests, you don’t need to switch antibiotics, and you don’t need to worry that something is wrong with your kidneys or bladder. Finish the course your doctor prescribed, stay hydrated, and expect your urine to return to normal within a day or two of your last dose.