Pus or thick, cloudy discharge from the penis is almost always a sign of urethritis — inflammation of the urethra — and the usual culprits are sexually transmitted infections: gonorrhea, chlamydia, or nongonococcal urethritis (NGU). Until proven otherwise, treat penile pus as an STI and get tested, because these conditions look too alike to call by sight.
Neisseria gonorrhoeae
Chlamydia trachomatis
Urethritis not caused by gonorrhea
| Item | Value |
|---|---|
| Gonorrhea | curable — Neisseria gonorrhoeae |
| Chlamydia | curable — Chlamydia trachomatis |
| Nongonococcal urethritis (NGU) | curable — Urethritis not caused by gonorrhea |
The short list of likely causes
When pus or a cloudy, thick fluid appears at the tip of the penis, a clinician's first thought is bacterial urethritis. The discharge is the visible product of white blood cells flooding into the urethra to fight an infection, which is why it looks milky, yellow, or green rather than clear. The most common causes are:
- Gonorrhea, which classically produces the thickest, most obviously purulent discharge.
- Chlamydia, which often causes thinner discharge or none at all.
- Nongonococcal urethritis (NGU), a catch-all for urethritis that isn't gonorrhea — frequently chlamydia, but also other organisms or no identifiable cause.
- Less commonly, a non-infectious irritation of the urethra that's still healing.
- Pus from the penis is almost always an STI until a test proves it isn't CDC STI Treatment Guidelines, 2021.
Which STIs cause pus or thick discharge from the penis
Each of these infects the urethra and triggers discharge, but the texture, color, and likelihood of symptoms differ. None of these distinctions is reliable enough to diagnose yourself; they only tell you what to look for and why a test matters.
Gonorrhea
Gonorrhea is caused by the bacterium Neisseria gonorrhoeae, which can infect the genitals, rectum, and throat CDC, About Gonorrhea. In men, it's the textbook cause of frankly purulent discharge — a white, yellow, or green fluid — usually paired with burning when urinating, and less often with swollen or painful testicles. Testicular pain points to epididymitis, inflammation of the coiled tube behind the testicle that stores and carries sperm, which can threaten fertility if left untreated. When the discharge is heavy and obviously pus-like, gonorrhea is high on the list. Read more in our gonorrhea overview.
Chlamydia
Chlamydia is caused by Chlamydia trachomatis, and most US genital infections are the serovars labeled D through K CDC Chlamydia Fact Sheet. It's notorious for being silent: roughly half of infected men have no symptoms at all. When symptoms do show up, they usually appear within one to three weeks of exposure, and the discharge tends to be thinner and clearer than gonorrhea's, more mucoid than frankly purulent. Chlamydia can also produce cloudy discharge and burning, so you can't separate it from gonorrhea by appearance. Our chlamydia guide covers it in depth.
Nongonococcal urethritis (NGU)
NGU isn't a single germ. It's the diagnosis when the urethra is inflamed but gonorrhea has been ruled out CDC Urethritis & Cervicitis, 2021. The causes vary: chlamydia is the most common identified one, but Mycoplasma genitalium (a tiny, slow-growing bacterium), sometimes Trichomonas, herpes simplex virus, or adenovirus can be responsible. In about half of cases, no organism is ever pinned down. NGU shows up mainly in men, and the discharge can be either mucoid (clear and stringy) or purulent (thick and pus-like), often with painful urination and an itchy or irritated feeling inside the urethra. Some men have no symptoms at all. On whether it resolves on its own, see can ngu go away on its own without treatment?
When it's not an STI
Not every discharge means an infection you caught from a partner. A urethra that's been irritated by aggressive cleaning, a recent catheter, friction, or a chemical exposure can leak fluid while it heals, and that drainage may look cloudy without any organism behind it. This is uncommon. Frank pus from the penis is treated as an STI until testing says otherwise, so don't talk yourself out of getting checked because you think it might be irritation.
How to tell them apart
You mostly can't, not on your own. Gonorrhea trends toward thicker, more colorful pus and chlamydia toward thinner discharge, but the overlap is huge and several of these infections are frequently silent. A lab test settles which one it is, if any. Self-diagnosis fails because the symptoms overlap, and a lab result turns a guess into an answer.
Clinicians lean on a few discriminating clues, but only to decide what to test and treat, never to skip the test:
- Discharge character: heavy yellow-green pus leans gonorrhea; thin, clear, or stringy mucus leans chlamydia or NGU.
- Timing: chlamydia symptoms, when they appear, usually come one to three weeks after exposure.
- Associated symptoms: testicular swelling and pain raise concern for epididymitis and warrant prompt care.
- Whether symptoms exist at all: silence doesn't rule anything out, since a large share of chlamydia infections and some NGU cases cause no symptoms.
Side-by-side comparison
| Feature | Gonorrhea | Chlamydia | NGU |
|---|---|---|---|
| Cause | N. gonorrhoeae | C. trachomatis | Chlamydia, M. genitalium, others, or unknown |
| Typical discharge | White, yellow, or green; often frank pus | Thinner, mucoid; sometimes none | Mucoid or purulent |
| Burning on urination | Common | Possible | Common |
| Often silent? | Can be | Yes — about half of infected men | Some cases asymptomatic |
| Symptom timing | Not stated on official guidance reviewed | Usually 1–3 weeks if symptoms occur | Variable |
| Confirmed by | NAAT | NAAT | Objective urethritis evidence plus NAAT |
How it's tested
For both gonorrhea and chlamydia, a nucleic acid amplification test (NAAT) is the recommended method, with sensitivity usually above ninety percent and specificity around ninety-nine percent for gonorrhea CDC Gonorrhea. NGU takes more: it requires objective proof of urethral inflammation — a Gram stain showing white blood cells without gonococci, or leukocyte esterase / white cells on a first-void urine sample — plus NAAT testing for chlamydia, gonorrhea, and, in stubborn cases, M. genitalium. In practice that means a urine sample, a self-collected swab, or a quick exam depending on what's suspected. You can get tested at a clinic or at home, and timing matters — see when to test after exposure so you don't test too early to catch it.
What to do next
Stop sexual contact until you've been evaluated, and get a NAAT-based test rather than guessing from the discharge. These are bacterial infections that respond to antibiotics, and treatment is straightforward once you know what you're treating, but the regimen differs by infection, so the test comes first. Notify recent partners so they can be checked too, since reinfection from an untreated partner is common. Testing is widely accessible and often free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days.
Red flags — when to get seen urgently
Most penile discharge can be handled at a routine clinic visit, but some symptoms shouldn't wait:
- Swollen, painful, or tender testicles, which may signal epididymitis and risk to fertility.
- Fever, chills, or pain in the lower abdomen or back, suggesting infection has spread beyond the urethra.
- Inability to urinate, or severe pain with urination.
- Blood in the urine or discharge.
- Rapidly worsening pain, redness, or swelling of the penis or scrotum.