The most common STIs that cause penile discharge are gonorrhea and chlamydia. A third infection, caused by a parasite called trichomoniasis, and a bacterium called Mycoplasma genitalium also produce discharge, though less frequently. The color, thickness, and timing of the discharge differ depending on the infection, and in many cases, more than one infection is present at the same time.
Gonorrhea: Thick, Yellow-Green Discharge
Gonorrhea is the STI most strongly associated with noticeable penile discharge. The discharge is typically heavy, yellow-green or yellowish, and visibly purulent, meaning it looks like pus. It can also appear brown or blood-tinged. Symptoms usually show up fast: anywhere from 1 day to 2 weeks after exposure. Many men notice the discharge on their underwear before they notice anything else.
Painful urination is the other hallmark symptom. Some men also develop urinary frequency or a burning sensation at the tip of the penis. Despite gonorrhea’s reputation for producing obvious symptoms, asymptomatic cases are more common than most people realize. In one multicountry study tracking new infections over two years, roughly 85 to 90 percent of men with confirmed gonorrhea reported no symptoms at all. That means a lack of discharge does not rule out infection.
Chlamydia: Mild, Clear or White Discharge
Chlamydia tends to be subtler. When it produces discharge, the fluid is usually thin, clear, or whitish, sometimes described as mucoid. The volume is lower than with gonorrhea, and some men mistake it for normal pre-ejaculatory fluid. Symptoms, when they appear, typically develop 7 to 21 days after exposure, so there is a longer window between sex and the first sign of trouble.
Chlamydia is the most common bacterial STI, and its silent nature is a major reason it spreads so effectively. The same multicountry study found that over 94 percent of men with incident chlamydia infections had no symptoms whatsoever. In some study populations, the figure was 100 percent. Painful urination may accompany the discharge, and in men under 35, chlamydia is the leading infectious cause of epididymitis, a painful swelling of the tissue behind the testicle.
Mycoplasma Genitalium and Trichomoniasis
When standard tests for gonorrhea and chlamydia come back negative but discharge is still present, two other infections are the usual suspects. Mycoplasma genitalium accounts for 15 to 25 percent of non-gonococcal urethritis cases in the United States. It produces symptoms similar to chlamydia: mild discharge, burning during urination, and sometimes no symptoms at all. It is not included in routine STI panels, so it often goes undiagnosed unless a clinician specifically orders testing for it.
Trichomoniasis, caused by a parasite rather than a bacterium, is another possibility. The CDC estimated over two million trichomoniasis infections in the United States in 2018, though the infection is more common in women. Only about 30 percent of infected people develop symptoms. In men, those symptoms can include a thin discharge from the penis, irritation inside the penis, and mild burning after urination or ejaculation.
How to Tell Infections Apart
Discharge appearance gives clues, but it is not reliable enough for diagnosis on its own. The general pattern looks like this:
- Heavy, yellow-green, pus-like: more typical of gonorrhea
- Light, clear or whitish, watery: more typical of chlamydia or Mycoplasma genitalium
- Thin, mild: more typical of trichomoniasis
In practice, discharge can be cloudy, brown, green, white, yellow, or blood-tinged regardless of the pathogen. Co-infections are common, particularly gonorrhea and chlamydia together, which muddies the picture further. The only way to identify the cause is testing. The gold standard is a nucleic acid amplification test (NAAT), which can be run on a urine sample or a urethral swab. Most clinics test for gonorrhea and chlamydia simultaneously. If those come back negative, a second round of testing for Mycoplasma genitalium and trichomoniasis is typically the next step.
Symptoms That Accompany Discharge
Penile discharge from an STI rarely occurs in isolation. The most common companion symptom is dysuria, a burning or stinging sensation during urination. You may also notice urinary frequency, the feeling of needing to urinate more often than normal, or suprapubic discomfort, a dull ache just above the pubic bone.
If the infection moves beyond the urethra, symptoms escalate. Epididymitis causes swelling, warmth, and pain in one or both testicles. In more advanced cases, the testicle itself becomes involved (epididymo-orchitis), which can cause fever, chills, and significant scrotal pain. Redness or crusting around the urethral opening is another sign that infection has been present for some time.
What Happens if You Don’t Get Treated
Untreated urethral infections can climb deeper into the reproductive tract. The most significant risk for men is epididymitis, which can permanently damage the tubes that carry sperm. Research on men who developed acute epididymitis found that about 10 percent had persistent azoospermia afterward, meaning no sperm in their ejaculate, and another 30 percent had significantly reduced sperm counts. Sperm concentrations typically recover within 3 to 6 months when treatment is prompt, but delayed treatment raises the risk of permanent obstruction or damage to sperm production.
Testicular atrophy, where the testicle physically shrinks and loses its ability to produce sperm, is a rare but serious complication of chronic infection. Beyond fertility, untreated STIs also increase your risk of transmitting HIV and of passing the infection to sexual partners, many of whom may develop their own complications.
Non-STI Causes Worth Knowing About
Not every case of penile discharge is sexually transmitted. Prostatitis, an inflammation of the prostate gland, can produce discharge alongside pelvic pain, difficulty urinating, or a frequent urge to urinate. Bacterial prostatitis sometimes develops after urinary tract infections, catheter use, or prostate procedures. Urinary tract infections in men, while less common than in women, can also cause cloudy or slightly discolored discharge from the urethra.
Chemical irritation from soaps, spermicides, or antiseptics can inflame the urethra enough to produce a small amount of clear discharge. The distinguishing factor is usually context: if discharge appeared after a new sexual partner or unprotected sex, an STI is the most likely explanation. If it followed a medical procedure or coincided with urinary symptoms without recent sexual exposure, non-infectious causes deserve consideration. Either way, testing is the fastest way to get a clear answer and appropriate treatment.