Spider Bite or Melanoma? How to Tell Them Apart

Most bites from common household spiders cause a small, mildly painful red bump, sometimes with localised swelling and itching, much like other insect bites. They peak within a day or two and settle over several days to two weeks. A medically significant bite is uncommon, and most people never see the spider responsible.

A bite is a self-limited reaction. Even bites that blister or form a small sore are on a healing trajectory: they get better, not worse, over the following two weeks. The one nuance is that genuine bites from certain spiders can cause a slow-healing ulcer, which is exactly why a non-healing lesion should not be assumed to be a bite indefinitely.

The cancers most often mislabelled as spider bites are basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and amelanotic (non-pigmented) melanoma. BCC can look like a pearly pink bump with a central dip that scabs, bleeds, and recurs. SCC can look like a scaly, crusted, tender sore. Amelanotic melanoma can be a pink or red nodule with little or no brown pigment, which is why it is so easily mistaken for an inflamed bite.

The theme across all of them is non-healing. A bite heals. A skin cancer mistaken for a bite sticks around for weeks to months, often slowly enlarging, scabbing and re-bleeding in cycles rather than steadily improving.

Spider bite: improves within days to two weeks. Even an angry-looking bite is clearly better by the two-week mark.

Skin cancer: does not resolve. A 'bite' that is no better — or larger, more raised, or more crusted — at 3-4 weeks is the single most important warning sign. The persistence-past-a-month rule catches a large share of skin cancers that were initially dismissed.

Spider bite: may scab once as it heals, then the scab falls off and the skin closes.

Skin cancer: scabs, partially heals, then breaks down and bleeds again — repeatedly. This recurring bleed-scab-non-healing cycle is one of the most reliable signals across BCC, SCC, and melanoma. A sore that 'almost heals' over and over but never fully closes needs evaluation.

Spider bite: usually a uniform red bump or a simple sore with a clean edge that contracts as it heals.

Skin cancer: look for a pearly or translucent rim with tiny visible blood vessels (BCC), a rolled or raised border around a central crater, persistent scale or crust (SCC), or irregular brown-black pigment at the edges (melanoma). Edges that are raised, rolled, or shiny rather than simply inflamed are a clue this is not a bite.

Spider bite: usually has a plausible story and a clear onset — you felt it, or it appeared overnight after sleeping somewhere spiders might be.

Skin cancer: the 'bite' explanation is often invented after the fact for a lesion with no clear origin, frequently on chronically sun-exposed skin (face, ears, scalp, forearms, lower legs) in someone over 50. If you never saw a spider and the lesion is on sun-damaged skin and not healing, the prior probability shifts toward skin cancer.

Seek prompt medical care for a genuine bite if: the area becomes rapidly more red, hot, swollen, and painful (cellulitis); red streaking spreads from the site; you develop fever, chills, nausea, or muscle aches; or a dark, expanding ulcer with a necrotic centre develops. These can indicate infection or a medically significant bite.

See a dermatologist for the lesion if: a 'bite' has not healed in 4 weeks; it scabs and re-bleeds in cycles; it has a pearly, rolled, or scaly border; it is a persistent pink, red, or pigmented nodule; or it is on sun-damaged skin in an older adult. Early BCC and SCC are highly curable, and the point of acting early is to keep treatment small.