My nails are breaking right in the middle. they are really flexible too and weak. been havig vitamin d and biotin but nothing yet. what should i do?

Here's 2 ideas: Two possibilities come to mind. Having low thyroid hormone level can cause this: other symptoms would be cold hands & feet, dry itchy skin, dry brittle hair, low temperature, weight gain. If you have these symptoms see your doctor about thyroid treatment. Another possibility is deficent sulfur. Try taking msm 1000mg/day and if that's the issue, your nails will start to grow in stronger within a mo.