Hard, sore lump under the skin: what to know

Tenderness often reflects inflammation or pressure in the surrounding tissue. A boil is a painful infection involving a hair follicle and may become red, warm or filled with pus. An epidermoid cyst can feel like a firm, rounded sac beneath the skin and may become sore when inflamed. A lump after a knock may represent local swelling or a small collection of blood.

Location, speed of growth, colour, mobility and surface changes help a clinician narrow the cause. Most lumps are not cancerous, but a firm lesion that keeps enlarging, bleeds, forms a recurring crust or does not heal should be assessed. Basal cell carcinoma can sometimes resemble a persistent skin growth rather than a painful lump.

Do not squeeze, pierce or repeatedly press the lump, because this can spread infection and worsen inflammation. Keep the area clean, avoid friction from tight clothing, and use a warm, moist compress for short periods if it is tender. Note its size, exact location and any changes in redness, drainage or pain. Arrange a primary-care or dermatology appointment when it is new and persistent, repeatedly returns, or interferes with movement or sleep.

Treatment depends on the cause. A clinician may recommend drainage for a collection of pus, removal of a cyst, or another targeted approach. Some inflamed raised lesions may be treated with a steroid injection into the skin when clinically appropriate; this should only be given after assessment.

Seek urgent medical care if the lump is rapidly becoming more painful or swollen, redness is spreading, pus is accompanied by fever, or you feel faint or unusually unwell. Prompt assessment is also sensible for a lump near the eye, nose or spine, or for someone with a weakened immune system.

Book a clinical review if it remains hard or sore, continues to grow, returns after settling, or develops bleeding, ulceration, a persistent crust, numbness or reduced movement. Do not wait for pain to appear before arranging review: some concerning skin growths are not tender.

Ask what features suggest infection, a cyst, an injury-related swelling or another growth; whether the lump needs drainage, imaging, sampling or removal; what changes should trigger faster review; and how to care for the area while you wait.