Nail biology sets this hub's recurring theme: patience is the active ingredient. A fingernail takes about six months to grow out fully, a toenail twelve to eighteen — so every genuine treatment, prescription or otherwise, is judged on the fresh nail emerging at the base months later, and every product promising visible transformation in weeks is promising something nails cannot do.
The territory splits in two. Fungal nail infection — thickened, yellowed, crumbling toenails, affecting perhaps 1 in 10 adults and half of over-70s — is a genuine infection with genuinely effective prescription treatment, a modestly useful topical tier, and a supplement-and-oil marketplace whose claims outrun its trials by the widest margin on this site. Brittle, ridged or discolored nails are mostly cosmetic biology — water and detergent cycles, aging, manicure chemistry — with a short, specific list of medical causes (thyroid, iron, psoriasis) worth knowing, and one famous supplement (biotin) whose reputation exceeds its evidence and whose habit can quietly corrupt blood tests.
Both guides end the same honest way: what nails need most is protection and time, what infections need is correct diagnosis before any treatment, and what the aisle sells is largely the impatience the biology forbids.