Genital swelling can be caused by a handful of STIs — most often genital herpes, chancroid, and lymphogranuloma venereum (LGV) — but plenty of non-STI problems like allergic reactions, cysts, and blocked glands cause it too. These conditions overlap too much to tell apart by sight, so a test is what tells you which one (if any) it actually is.
painful blisters that crust over; tends to recur
painful, soft, ragged ulcer(s)
Chlamydia trachomatis serovars L1, L2, or L3
| Item | Value |
|---|---|
| Genital herpes | managed — painful blisters that crust over; tends to recur |
| Chancroid | curable — painful, soft, ragged ulcer(s) |
| Lymphogranuloma venereum (LGV) | curable — Chlamydia trachomatis serovars L1, L2, or L3 |
The short list: what causes genital swelling
When swelling shows up around the genitals, groin, or rectum, a clinician runs through a fairly small differential. On the STI side, three infections account for most cases worth worrying about: genital herpes, chancroid, and LGV. Each produces a slightly different pattern. Herpes tends toward painful blisters and sores with tender glands, chancroid toward deep painful ulcers with pus-filled groin nodes, and LGV toward one-sided groin swelling or rectal inflammation.
None of these reliably announces itself. Many cases are mild or silent, and the visible signs blur together. A swollen lymph node feels like a swollen lymph node whether the cause is a virus, a bacterium, or an ingrown gland. So self-diagnosis fails here.
Which STIs cause genital swelling
Genital herpes
Genital herpes is caused by two viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC, About Genital Herpes. Most people who carry it have no symptoms or very mild ones, don't know they're infected, and the majority of HSV-2 infections go undiagnosed. The textbook outbreak is the minority experience.
When symptoms do show, a first outbreak is the most dramatic: small blisters that break open into painful sores, which can take a week or more to heal, often with flu-like symptoms — fever, body aches, and swollen glands. The sores can appear on or around the genitals, rectum, or mouth. The "swelling" people notice is usually those tender, enlarged lymph nodes in the groin or the puffy, inflamed skin around a cluster of sores. Later outbreaks are shorter and milder, and some people get a warning prodrome — tingling, itching, or burning — a day or so before lesions appear.
There's no cure, but daily or episodic antiviral medicine shortens outbreaks and lowers transmission. Some people also ask about supportive measures and natural approaches — see our overview of alternative herpes treatments for what's reasonable and what isn't.
Chancroid
Chancroid is a bacterial STI caused by Haemophilus ducreyi CDC, Chancroid, 2021. It's become rare in the United States, but it's a classic cause of painful genital ulcers paired with tender, swollen, pus-filled lymph nodes in the groin. Look for one or more deep, painful ulcers plus those inflamed nodes. The pain itself is a clue: the syphilis chancre is famously painless while the chancroid ulcer hurts.
If you have a painful sore with groin swelling, the appearance alone won't seal the diagnosis. Read more on the full picture of chancroid symptoms to see how it differs from look-alikes.
Lymphogranuloma venereum (LGV)
LGV is an aggressive form of chlamydia — caused by Chlamydia trachomatis serovars L1, L2, or L3, which are different from the serovars behind ordinary genital chlamydia and drive much more severe inflammation CDC, LGV Treatment Guidelines. That inflammation is why LGV shows up as swelling.
There are two main presentations. The more common one overall is proctocolitis from rectal infection: mucoid or bloody rectal discharge, anal pain, constipation, fever, or tenesmus — a constant, frustrating urge to pass stool. Outbreaks have clustered among men who have sex with men, often with HIV. The second is tender, usually one-sided swelling of the lymph nodes in the groin (inguinal or femoral) that can progress to buboes — fluctuant, pus-filled swellings — and this is the more typical picture in heterosexual patients.
Because LGV can quietly damage tissue if untreated, prompt diagnosis matters. Our guide to lgv symptoms walks through the rectal and groin patterns in more detail.
When it's NOT an STI
Swelling down there is often something completely unrelated to sex. An allergic reaction — to a new soap, lubricant, latex condom, or laundry detergent — can leave skin puffy, red, and itchy. A cyst (a fluid-filled lump under the skin) or a blocked gland, such as a Bartholin's gland near the vaginal opening, can produce a tender, localized swelling that has nothing to do with an infection you caught from a partner.
Ingrown hairs, simple irritation from shaving, and minor trauma round out the common harmless causes. This isn't about talking yourself out of testing. Swelling alone doesn't equal STI, and the only way to know is to look at the whole picture and test.
How to tell them apart
You usually can't tell these apart by sight. A few discriminating features point the way and guide which test to run.
- Painful blisters that break into sores, often in clusters, with tingling beforehand — points toward herpes.
- A deep, painful ulcer plus a pus-filled groin node — points toward chancroid, especially when syphilis and herpes tests come back negative.
- One-sided groin swelling progressing to a bubo, or rectal pain with discharge and tenesmus — points toward LGV.
- Itchy, red, puffy skin after a new product, with no ulcer — points toward an allergic reaction rather than an STI.
- A single tender lump near the vaginal opening, no sores — points toward a cyst or blocked gland.
These patterns are clues, not verdicts. Several of these infections are frequently silent, and the symptoms overlap too much to settle by appearance, so a test is what tells you which one it is, if any.
Side-by-side comparison
| Cause | Typical swelling pattern | Pain? | Sores/ulcers | How it's confirmed |
|---|---|---|---|---|
| Genital herpes | Tender swollen glands; inflamed skin around sores | Yes, sores are painful | Blisters that break into sores | Swab of a lesion (NAAT or culture) |
| Chancroid | Tender, pus-filled groin nodes | Yes, ulcer is painful | One or more deep ulcers | Special culture for H. ducreyi; clinical picture with negative syphilis/herpes |
| LGV | One-sided groin swelling/buboes, or rectal inflammation | Tender nodes; anal pain | Often no obvious ulcer by the time swelling appears | Chlamydia NAAT at the symptomatic site |
| Allergic reaction | Diffuse puffy, itchy skin | Itchy more than painful | None | History (new product); resolves when removed |
| Cyst / blocked gland | Single localized lump | May be tender | None | Exam by a clinician |
How it's tested
Testing depends on what's suspected: confirming herpes when lesions are present means swabbing a sore for type-specific virologic testing by NAAT or culture CDC, Herpes Testing; LGV is diagnosed with a chlamydia NAAT taken at the symptomatic site, such as the rectum, with other causes ruled out; and chancroid is a probable diagnosis when you have painful ulcers, a typical appearance, and negative syphilis and herpes results, with definitive proof requiring a special H. ducreyi culture that isn't widely available CDC, Genital Herpes Tx Guidelines. Don't guess — get tested.
In practice, testing is simpler than people fear: a urine sample, a self-collected swab, or a quick exam depending on what's being checked. It's free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. If you're testing soon after a possible exposure, timing matters — see when to test after exposure so you don't test too early to catch it.
What to do next
If you have new genital swelling, sores, or groin lumps, get checked rather than wait it out. Herpes, chancroid, and LGV all have effective treatments, and the sooner LGV in particular is treated, the less tissue damage it causes. Once you have a diagnosis, your clinician will match the medicine to the specific infection — the regimens differ enough that the right drug depends entirely on which one you have.
Red flags — when to get seen urgently
- Rapidly spreading swelling, severe pain, or skin that's hot and darkening — get emergency care.
- High fever with genital or groin swelling.
- A bubo that is large, very painful, or draining pus.
- Bloody rectal discharge with anal pain and fever.
- Trouble urinating because of swelling, or any swelling that blocks the urethra.
- Signs of a severe allergic reaction — facial or throat swelling, trouble breathing — which is a 911 situation.