Usually polish staining or nail fungus - thick, crumbly, one-sided yellow nails are the fungus pattern. All nails yellowing together, or yellow nails with diabetes, deserve a medical look.
What to do
Common questions
What does it mean if my nails are yellow?
Yellow nails usually have a cosmetic explanation: dark polish worn for weeks stains the nail plate yellow, and it grows out with the nail. A polish break of a few weeks, plus gentle buffing, tells you if staining was the cause. The other common cause is toenail fungus - thick, yellow, crumbly, sometimes lifting nails, usually starting at one edge of one toenail and creeping. MedlinePlus notes fungal nail changes are common, stubborn, and slow: over-the-counter treatments rarely clear established cases, and prescription antifungals work but take months, because you are waiting for clean nail to grow out. A clinician can confirm fungus with a quick nail clipping test before you commit to treatment - worth it, since thick nails from psoriasis or old injuries look similar. The pattern to know for the doctor visit: all twenty nails yellowing and thickening together with slow growth can accompany lung conditions or lymphedema - rare, but that is why symmetric, everywhere-yellow nails deserve a mention. And yellow nails in someone with diabetes warrant extra care, because fungus plus diabetes raises foot-complication stakes. Pymander is a free AI doctor, 24/7 by text. Describe which nails and how they changed, and it can help you tell stain from fungus.