References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
A swollen testicle is usually caused by an infection, injury, or fluid buildup, not cancer. That said, any new or unexplained testicular swelling should be checked by a doctor, especially if it came on suddenly or is painful. Some causes need urgent treatment.
This guide covers the most common causes of testicular swelling, which STIs can cause it, the warning signs of testicular torsion (a medical emergency), and when you need to see a doctor.
Could it be an STI? Get a discreet home test kit
Common Causes of a Swollen Testicle
| Cause | Pain? | Speed of Onset | Other Symptoms | Urgency |
|---|---|---|---|---|
| Epididymitis (infection) | Yes, gradual | Days | Redness, warmth, discharge possible | See GP within 24-48 hours |
| Orchitis (testicular inflammation) | Yes | Days | Fever, nausea, swollen groin glands | See GP within 24-48 hours |
| Testicular torsion | Severe, sudden | Minutes | Nausea, vomiting, testicle rides high | A&E IMMEDIATELY |
| Hydrocele (fluid buildup) | Usually painless | Weeks to months | Soft, fluid-filled swelling | GP routine |
| Inguinal hernia | Aching, worse with activity | Variable | Lump in groin, worse when standing | GP within a week |
| Varicocele (enlarged veins) | Aching or dragging | Gradual | Feels like “bag of worms” above testicle | GP routine |
| Testicular cancer | Usually painless | Weeks to months | Hard lump ON the testicle | GP within 2 weeks |
| Injury/trauma | Yes | Immediate | Bruising, tenderness | A&E if severe; GP if mild |
STIs That Cause Testicular Swelling
Epididymitis from chlamydia or gonorrhoea
The most common STI-related cause of a swollen testicle in men under 35 is epididymitis, an infection of the epididymis (the coiled tube behind each testicle that stores and carries sperm).
In sexually active men under 35, chlamydia and gonorrhoea are the most common causes. In men over 35, urinary tract bacteria (E. coli) are more common.
Symptoms of epididymitis:
- Gradual pain and swelling on one side of the scrotum
- Redness and warmth of the affected side
- Pain that may spread to the lower abdomen or groin
- Penile discharge (if caused by chlamydia or gonorrhoea)
- Pain or burning when urinating
- Low-grade fever
Treatment: antibiotics. If chlamydia is the cause, doxycycline for 10-14 days. If gonorrhoea, ceftriaxone injection plus doxycycline. Rest, scrotal support (supportive underwear), and pain relief with ibuprofen are also recommended.
Untreated epididymitis can lead to an abscess, chronic pain, or reduced fertility. Get tested and treated promptly.
Orchitis from mumps or other infections
Orchitis (inflammation of the testicle itself) can be caused by the mumps virus or by bacterial infections that spread from the epididymis. Mumps orchitis typically occurs 4-7 days after the facial swelling of mumps and affects one or both testicles.
STI-related orchitis is less common than epididymitis but can occur when epididymitis is left untreated and the infection spreads to the testicle.
Testicular Torsion: The Emergency
Testicular torsion is NOT an STI. It happens when the testicle twists on its spermatic cord, cutting off blood supply. It requires emergency surgery within 4-6 hours to save the testicle.
Warning signs of torsion:
- Sudden, severe pain in one testicle that comes on within minutes
- The affected testicle may sit higher than normal or at an unusual angle
- Nausea and vomiting
- Abdominal pain
- Most common in boys and young men aged 12-25
If you suspect torsion, go to A&E immediately. Do not wait to see your GP. Delay risks losing the testicle.
When to See a Doctor
- A&E immediately: Sudden severe testicular pain (possible torsion)
- GP within 24-48 hours: Gradual swelling with pain, redness, warmth, discharge (possible epididymitis)
- GP within 2 weeks: Painless hard lump on the testicle (needs cancer screening)
- GP routine: Painless soft swelling, heaviness, or aching (likely hydrocele or varicocele)
If you have testicular swelling with discharge or pain urinating, get tested for STIs. Chlamydia and gonorrhoea are treatable with antibiotics, but delayed treatment can cause lasting damage.
Order a home STI test or find your nearest sexual health clinic.
Read more: What is chlamydia? | What is gonorrhoea? | Chlamydia in men
Frequently Asked Questions
Can chlamydia cause a swollen testicle?
Yes. Chlamydia can cause epididymitis, which leads to swelling, pain, and redness on one side of the scrotum. It’s the most common cause of testicular swelling from an STI in men under 35.
Is a swollen testicle always serious?
Not always. Hydroceles and varicoceles are common and usually harmless. But sudden severe pain (possible torsion) and painless hard lumps (possible cancer) both need prompt medical attention. When in doubt, see a doctor.
Can a swollen testicle affect fertility?
Potentially. Untreated epididymitis can cause scarring that blocks sperm flow. Testicular torsion can damage the testicle permanently if not treated within hours. Varicoceles can affect sperm quality in some men. If you’re concerned about fertility, see your GP for assessment.
How long does epididymitis take to heal?
With antibiotics, symptoms usually improve within 48-72 hours, though full resolution can take 2-4 weeks. Swelling may persist for several weeks even after the infection has cleared. Complete the full course of antibiotics and follow up with your doctor if symptoms don’t improve.
Related reading
- Is Testicular Pain a Sign of an STI?
- What Is a Female (Internal) Condom?
- Blood Test for Syphilis, Hepatitis B and Hepatitis C
- Can You Have Chlamydia and Test Negative?
- Can You Test for STIs While on Your Period?
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
