For toenails that are tinged yellow but smooth to the touch, dark nail polish could be to blame. To eliminate stains, New Orleans dermatologist Deirdre O'Boyle Hooper, MD, recommends lightly buffing your (unpolished) nails and soaking them for 5 to 10 minutes in lemon juice or, if you're more ambitious, a slushy mixture of two parts peroxide and one part baking soda every other day until you see an improvement. And while vertical lines and brittle nails are both normal signs of aging, toenails that are thick and discolored could be caused by a fungal infection. Treating fungus with a topical prescription treatment rarely works because it's difficult for medicine to penetrate the nail plate, says O'Brien. She adds that taking prescription oral Lamisil is the most effective option (but has been associated with side effects including liver damage).
Keep in mind results won't come quickly—it can take up to a year for toenails to fully grow out.