Nail strengtheners have their place, but Dr Stern suggests avoiding them if your manicure doesn’t stick around. Why? She says that some products on the market contain formaldehyde or formalin, which can cause irritation and reactions. The FDA reports that formaldehyde fuses with the keratin that appears naturally in nails, and this makes the nail harder. “Formaldehyde will initially harden the nail, however with time, the nail becomes paradoxically brittle,” Dr Stern explains. Here the nail may also be at risk of lifting or separating off of the nail bed, she says, an issue called onycholysis.