Short answer
Any new penile sore triggers same-day evaluation logic: abstain from all sexual activity immediately, get fresh-sore swabs plus syphilis blood testing promptly, and let laboratory results, not pain patterns, not internet photos, not hope, name the cause.
Sores that “look classic” mislead regularly, and swab accuracy decays with every day of healing, so speed is diagnostic yield.
Continue with Penile Discharge: When to Seek Testing and Urethritis: Causes, Symptoms, and Testing for the directly connected guidance.
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See Tests & Prices →STI patterns and their classic (unreliable) looks
- Herpes: grouped painful blisters becoming shallow ulcers, often with burning urination or groin tenderness.
- Syphilis: solitary painless firm chancre with clean base. The painlessness that causes dangerous delay.
- Chancroid (rare in the US): soft, painful, ragged ulcer with tender groin swelling.
- LGV: small transient sore followed weeks later by painful groin swelling.
Memorize these as orientations, never as diagnoses: real sores break textbook rules constantly.
Non-STI mimics that fool confidently
- Friction trauma and tears (vigorous or dry intercourse).
- Folliculitis and boils (hair-follicle infections at the base).
- Behçet’s aphthae (with oral ulcers and eye/joint features).
- Fixed drug eruptions (same-spot recurrence with each dose of a medication. The drug timeline is diagnostic).
- Psoriasis and eczema (scaling, chronicity, extragenal disease).
- Penile malignancy (persistent, atypical, non-healing lesions in older patients. Biopsy thresholds, not reassurance).
Workup specifics that maximize yield
Lesion mapping and photography (progression documented objectively).
Unroofed-lesion NAAT swabs with HSV typing. Intact blister fluid tests best, so don’t pop, pick, or cream sores beforehand.
Syphilis and HIV serology regardless of suspected cause.
Medication review for fixed-eruption patterns.
Urologic or dermatologic referral for atypical or persistent findings, with biopsy for non-healing lesions.
While awaiting results
Complete abstinence (including oral sex and sex with condoms. Sores shed regardless).
Keep the area clean and dry.
Don’t apply random creams that obscure morphology. List recent partners and symptom dates for the appointment.
Tell partners about possible exposure now; confirm specifics after results.
When to seek care
Same-day for any new sore. Healing proves nothing (chancres and herpes both remit spontaneously) while swab yield decays daily.
Urgently for inability to urinate, rapidly spreading pain and swelling, black or necrotic tissue, or immune suppression with severe lesions.