Why Does Methylene Blue Turn Your Urine Blue or Green?

Methylene blue turns your urine blue or green because your kidneys filter the dye out of your bloodstream and excrete it — it is a normal, harmless, and completely expected effect, not a sign that anything is wrong. The color usually shows up within a few hours of taking it and fades as the compound clears your system.

Why it happens

Methylene blue is, first and foremost, a dye — it was used to color textiles in the 1870s before its medical uses were discovered. When you take it, some of it is metabolized and some is excreted unchanged by your kidneys. As that pigment passes into your urine, it tints it blue or, mixed with normal yellow urine, green. It is the same reason the compound can temporarily stain your tongue.

Is blue or green urine harmful?

No. On its own, methylene blue turning your urine blue or green is harmless and temporary. It simply reflects the compound moving through and out of your body. It is one of the most predictable things about taking methylene blue.

How long does it last?

For most people the color appears within a few hours and clears within about a day, depending on your dose, hydration, and metabolism. Drinking water as usual helps everything move through normally.

When blue or green urine is worth a second look

If you are not taking methylene blue (or another known cause) and your urine turns blue or green, that is different — it can occasionally point to certain medications, dyes, or, rarely, an infection, and is worth mentioning to a clinician. But when you have just taken methylene blue, the color is the expected result, not a red flag.

What else methylene blue can stain

Because it is a strong dye, methylene blue can also temporarily stain teeth and skin and will mark fabric or worktops permanently. Taking it diluted in water and rinsing your mouth afterward helps. For dosing and measurement tips, see our dosage guide, and for the full picture of what to expect, our side effects overview.

Frequently asked questions

Does blue urine mean the methylene blue is working? It confirms the compound is being excreted, but it is not a measure of any specific benefit — just that it has passed through your system.

Can I prevent the color? Not really, and there is no need to. It is a normal effect of an intensely pigmented compound.

Should I use less to avoid it? Dose should be guided by research ranges and professional advice, not by urine color. See our USP pharmaceutical-grade methylene blue and the dosage guide for how to measure accurately.

These statements have not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.