What should you know about black nail fungus?

A black nail is not automatically fungal; infection, trapped blood, external staining and pigment-producing nail disorders can look similar. Fungal nail infection more often also causes thickening, crumbling, distortion or debris beneath the nail, but appearance alone cannot confirm it. A clinician may examine the nail and collect a clipping or scraping before recommending antifungal treatment.

Fungal infection can discolour a nail and alter its texture, particularly when the nail becomes brittle or separates from the nail bed. A dark patch can also be old blood after pressure or trauma, surface material, bacterial growth in a lifted nail, or pigment arising within the nail unit. More than one problem can occur together. The pattern, recent injuries, footwear, medicines and changes over time help narrow the cause.

Keep the nail clean and dry, trim it carefully without digging beneath it, and avoid sharing clippers or footwear. Do not cover an unexplained dark area with polish before an assessment, because the pattern may help with diagnosis. Photographing the nail in consistent light can show whether the mark moves outward as the nail grows. Avoid starting leftover antifungal tablets: oral medicines require an appropriate diagnosis and a review of health conditions and medicine interactions.

Arrange a prompt dermatology assessment for a new dark stripe or patch without a clear injury, widening or irregular pigment, colour affecting the skin beside the nail, repeated bleeding, or a nail that is being destroyed. Seek urgent care for spreading redness, warmth, swelling, pus, fever or severe pain. Earlier review is also sensible if you have diabetes, reduced circulation or a weakened immune system, because nail and surrounding-skin infections may need closer management.

Ask whether the colour and texture fit fungal infection or another nail disorder, whether a clipping or scraping would clarify the diagnosis, and which treatment suits the number and thickness of affected nails. It is also useful to discuss athlete's foot, previous nail trauma, current medicines, liver or kidney conditions, and how to prevent reinfection if fungus is confirmed.