A swollen testicle on one side has several possible causes, and not all are sexually transmitted. STIs like chlamydia and gonorrhea can inflame the tube behind the testicle and cause one-sided pain or swelling. But sudden, severe pain may signal testicular torsion, a surgical emergency. If onset is abrupt and intense, seek care immediately.
Chlamydia trachomatis
Neisseria gonorrhoeae
| Item | Value |
|---|---|
| Chlamydia | curable — Chlamydia trachomatis |
| Gonorrhea | curable — Neisseria gonorrhoeae |
Quick answer: what causes swelling in one testicle?
One-sided testicular swelling usually traces back to one of a handful of causes. The most time-critical is testicular torsion, where the spermatic cord twists and cuts off blood flow; it has to be ruled out fast. STIs are another common source, typically by triggering epididymitis, inflammation of the coiled tube behind the testicle that stores and carries sperm. The rest are non-infectious: injury, or a hydrocele, a painless fluid collection around the testicle. The short list:
- Testicular torsion — a surgical emergency where the cord twists; sudden, severe pain.
- STI-related epididymitis — most often from chlamydia or gonorrhea in sexually active people.
- Injury or trauma to the scrotum.
- Hydrocele — a soft, usually painless swelling from fluid buildup.
These causes overlap so much that you can't reliably sort them out by feel or appearance, and several STIs cause no symptoms at all. A test settles which one it is, if any.
Which STIs cause pain or swelling in one testicle?
Two bacterial STIs are the usual culprits behind STI-related testicular pain. Both can infect the urethra and travel up the reproductive tract to inflame the epididymis. When that happens, the swelling and tenderness are often felt on one side first.
Chlamydia
Chlamydia is caused by the bacterium Chlamydia trachomatis, and most US genital infections come from a group of strains called serovars D–K CDC Chlamydia. It's a notoriously 'silent' infection. Roughly half of infected men have no symptoms at all, so it can cause trouble before you notice. When men do have symptoms, they may include burning on urination or discharge. Less commonly, the infection ascends and inflames the epididymis, producing one-sided testicular pain and swelling. If symptoms appear, they usually show up within one to three weeks after exposure.
Gonorrhea
Gonorrhea comes from the bacterium Neisseria gonorrhoeae, which can infect the genitals, rectum, and throat CDC Gonorrhea. In men, the classic signs are burning when urinating and a white, yellow, or green penile discharge. Less commonly, gonorrhea causes swollen or painful testicles through the same epididymitis mechanism as chlamydia. Untreated, the inflammation can affect fertility, so one-sided swelling in a sexually active man should never be dismissed as a strain or a knock.
When it's NOT an STI
Plenty of one-sided swelling has nothing to do with sex. The three you most need to know:
- Testicular torsion — the spermatic cord twists and chokes off blood supply to the testicle. This is a surgical emergency: the longer the blood is cut off, the higher the chance the testicle can't be saved. Pain is typically sudden and severe, often with nausea or vomiting, and may strike out of nowhere, even during sleep.
- Injury — a direct hit or twist to the scrotum can cause swelling and bruising. The cause is usually obvious from the event itself, but a hard blow can sometimes mimic or even trigger torsion.
- Hydrocele — a collection of fluid around the testicle that produces a soft, usually painless swelling. It often develops gradually and feels like a balloon of water rather than a tender, hot lump.
How to tell them apart
You often can't, not by sight or feel alone. These conditions overlap too much, and several STIs are frequently silent. Clinicians do weigh a few discriminating features. Onset speed matters most: torsion tends to arrive abruptly and ferociously, while STI-related inflammation builds over hours to days and often comes with urinary symptoms or discharge. Torsion pain is rarely accompanied by discharge; epididymitis often is. A hydrocele is typically soft and painless. Injury has a clear backstory. None of this replaces an exam, but it's how a clinician starts narrowing things down.
Side-by-side comparison
| Cause | Onset | Pain level | Other clues | STI? |
|---|---|---|---|---|
| Testicular torsion | Sudden, often abrupt | Severe | Nausea/vomiting; can happen during sleep; emergency | No |
| Chlamydia (epididymitis) | Builds over hours–days | Mild to moderate | Often burning on urination or discharge; frequently silent | Yes |
| Gonorrhea (epididymitis) | Builds over hours–days | Mild to moderate | White/yellow/green discharge; burning urination | Yes |
| Injury | Immediate after event | Varies | Clear trauma history; bruising | No |
| Hydrocele | Gradual | Usually painless | Soft, fluid-filled swelling | No |
How it's tested
For the STI side, a nucleic acid amplification test (NAAT) is the preferred method for both chlamydia and gonorrhea, usually run on a urine sample or a self-collected swab CDC STI Treatment Guidelines, 2021; NAAT for gonorrhea has sensitivity above 90% and specificity around 99% CDC. Testing is often free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. For the full how-to, see how to get tested, and if your exposure was recent, check when to test after exposure so you don't test too early to catch it.
What to do next
If pain is sudden and severe, skip the testing question and go to an emergency room now; torsion is treated by the clock. If swelling is milder and building, get an STI test rather than guessing. If chlamydia or gonorrhea is confirmed, both are curable with antibiotics, and your recent partners need treatment too. Read more about chlamydia on the chlamydia page and about gonorrhea on the gonorrhea page.
Red flags — when to get seen urgently
Some symptoms can't wait for a clinic appointment. Get emergency care if you have any of these:
- Sudden, severe testicular pain — especially if it woke you from sleep or came on within minutes.
- Pain with nausea or vomiting.
- A testicle that sits higher than usual or at an odd angle.
- Rapidly worsening swelling, redness, or fever.
- Severe pain after an injury to the scrotum.