Hi there! Thank-you for your enquiry. It could be a number of things that causes this. Sometimes it’s as simple as genetics. Other times you’ll find that if you’re on any chronic medication or you suffer from an auto immune disease or your health is compromised in some way, your nails will be thin and “bendy”. I’m glad that you’re taking a multivitamin, but are you taking something specifically for your nails? You need to take a supplement that is high in calcium, magnesium and potassium. There are some great hair and nail supplements out there. Solal makes really great supplements; check out their website. Sometimes you’ll also find that your hair and nails and skin will be thin and weak if you are deficient in certain vitamins and minerals and then it requires you to simply supplement that particular component. You can have blood tests to ascertain if this is something you may suffer with. There are quite a few patients who suffer with weak nails who find that the simple stimulation from a monthly manicure is helpful in building up the nails again.