Green or Blue Urine: Medications, Dyes, and What It Means
Updated April 2026
Almost always medication or dye
Blue or green urine looks alarming but is almost always caused by a medication or medical dye. Review any medications or recent medical procedures. If no explanation, or if you have fever or pain, see your GP.
Why Urine Turns Blue or Green
Blue or green urine is rare, visually striking, and almost never a health emergency. The colour is produced when substances that absorb light at the red end of the spectrum are excreted through the kidneys - leaving the blue-green end of the spectrum visible.
The most common cause by far is a medication or medical dye. If you recently had a medical procedure, a colonoscopy, cystoscopy, or any procedure involving injected dyes, and you notice blue-green urine within 24 hours, the dye is the explanation. Similarly, certain medications are well documented to produce blue-green urine and this should be disclosed when they are prescribed.
The Mayo Clinic lists blue-green urine as primarily medication or dye-related and notes it is typically not cause for concern if the cause is known.
Medications and Substances That Cause Blue-Green Urine
Methylene blue
A dye used in medical procedures (sentinel lymph node mapping, certain diagnostic tests) and as an antidote for methaemoglobinaemia. Produces intensely blue urine within hours of administration. Expected and harmless.
Amitriptyline
Tricyclic antidepressant used for depression, nerve pain, and migraine prevention. Blue-green urine is a known side effect, though not universal. Does not indicate toxicity or need for dose change.
Propofol
An intravenous anaesthetic agent used for surgical sedation. Green urine (sometimes described as neon green) can occur during or after propofol infusion. This is a well-documented, harmless side effect.
Indomethacin
Non-steroidal anti-inflammatory drug (NSAID) used for arthritis, gout, and bursitis. Blue-green tint is a rare but documented effect.
Cimetidine
H2 blocker for stomach acid (e.g. Tagamet). Very rarely causes blue-green urine.
Food dye (FD&C Blue No. 1)
Brilliant Blue in large quantities from sports drinks, candy, or blue-coloured food can tint urine blue-green. Harmless and transient.
Pseudomonas aeruginosa UTI
Very rare. Pseudomonas, a bacterial pathogen, produces pyocyanin, a blue-green pigment. This is a serious infection requiring urgent treatment. Would be accompanied by fever, severe symptoms, and typically occurs in hospital patients or immunocompromised individuals.
Rare Genetic Causes
Blue diaper syndrome (familial benign hypercalcaemia with blue urine) is an extremely rare inherited condition in which tryptophan is not properly absorbed in the gut. Gut bacteria convert it to indoles, which are absorbed and converted by the liver to indican, which is then excreted in urine where it oxidises to indigo dye. This is typically diagnosed in infancy and is very rare.
The practical takeaway: if you are an adult with blue-green urine and no known medication or dye explanation, mention it to your GP, but the likely cause is still medication or food. Genetic causes are exceedingly rare.
When to Seek Care
Today: Blue-green with fever, severe urgency or pain, or if you are immunocompromised (possible Pseudomonas UTI - rare but requires urgent treatment).
Book GP: Blue-green with no medication or food explanation and no recent medical procedure.
Monitor: Recent medical procedure (colonoscopy, surgery under propofol, methylene blue dye injection) or known medication side effect - expected, will clear within 24-48 hours.
Frequently Asked Questions
Why is my urine blue-green after surgery?
Blue-green urine after surgery is usually from propofol (the most commonly used anaesthetic agent) or from methylene blue dye used during certain procedures (lymph node mapping, urological procedures). Both are expected, harmless, and clear within 24-48 hours.
Can amitriptyline cause blue or green urine?
Yes. Amitriptyline is a well-documented cause of blue-green urine. This is a known side effect and does not indicate toxicity or the need to stop the medication. Mention it to your GP or prescriber if you are concerned, but it is typically harmless.
Is blue urine dangerous?
In the vast majority of cases, no. Blue or green urine from medications or dyes is harmless and self-resolving. The scenario that does need urgent assessment is blue-green urine in an immunocompromised patient with fever - this could indicate a Pseudomonas UTI, which requires prompt antibiotic treatment.
What foods cause blue or green urine?
Blue food dye (FD&C Blue No. 1) in large quantities from sports drinks, blue candy, or heavily dyed foods can produce blue-green urine. This is harmless and clears within 24 hours of stopping the food.
Sources: Mayo Clinic; Cleveland Clinic.