Question: What causes lines or ridges in nails?

Dr. Beer's answer:
Aging is the most common cause of nail lines and ridges. Chemotherapy or kidney disease may also cause lines or ridges, and trauma to the nail is another common culprit.
Although there is no way to cure these, taking the supplement biotin every day and applying topical moisturizers such as AmLactin, Lac Hydrin, or Theraplex can help to strengthen the nail plate. You should be able to find these at your local drug store.
Also, ask your doctor to check your thyroid and iron level. If these are low, balancing them will sometimes helps the nails dramatically.
If your nails don't change or seem to be getting worse after trying these treatments for three or four months, a visit to your dermatologist may be in order.