Short answer
Penile discharge has a ranked differential, and the ranking matters because it sets testing order: STI organisms (gonorrhea, chlamydia, trichomoniasis, M.
genitalium, herpes urethritis) cause the large majority in sexually active men. Assume STI until testing proves otherwise.
UTIs, prostatitis, strictures, irritants, and (rarely, in older patients) urethral malignancy fill the meaningful minority.
One visit with NAAT plus urinalysis sorts both lists; character of discharge prioritizes but never diagnoses.
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See Tests & Prices →STI causes ranked by likelihood and look
- Gonorrhea: profuse yellow-green discharge with burning, days after exposure. The classic presentation, though milder cases occur.
- Chlamydia: scant clear-to-white discharge, sometimes barely noticeable; easily missed, equally consequential.
- Trichomoniasis: often with urethral irritation; frequently coexists with chlamydia.
- M. genitalium: the persistent case specialist. Suspect when discharge survives standard therapy.
- Herpes urethritis: burning-predominant with scant discharge, sometimes with visible lesions.
Co-infections are common enough that panels test broadly rather than chasing single organisms.
Non-STI causes ranked by context
- Bacterial UTI: frequency, urgency, cloudy urine. Dominant in older men, with prostate enlargement, stones, or instrumentation.
- Chronic prostatitis: perineal ache, recurrent bacteriuria, pain after ejaculation.
- Urethral stricture or foreign body: obstructive symptoms with colonization.
- Chemical or mechanical irritation: new soaps, spermicides, vigorous instrumentation.
- Neoplastic bleeding: older patients, bloody persistent discharge. Urologic referral without delay, though rare.
Workup in one visit
Exposure and symptom history (timing, partners, prior episodes), genital exam, first-catch urine NAAT (gonorrhea/chlamydia/trich), urinalysis with culture, prostate assessment where age or symptoms suggest, HIV/syphilis screening by risk.
Syndromic therapy starts same-visit per guidance. Treatment begins on probability while results confirm, then adjusts.
See Burning Urination in Men: UTI vs STI.
When to seek care
Promptly for any discharge (STI presumption until tested protects partners as well as patients); with age or urinary patterns suggesting UTI (dual workup, not either/or); urgently for bloody discharge with obstruction, fever, or inability to urinate.