Nail strengtheners work increasing the amount of cross linking within the nail plate. More cross-links = more strength.
Most people automatically think that is what they need... 'strength'. In all reality, they want 'toughness'. What is the difference? Toughness is a combination of strength and flexibility which enables nails to absorb shock without tearing or breaking. The stronger you make something, the less flexible it becomes and therefore the more brittle it will also become.
Clients who overuse natural nail strengtheners frequently have problems with natural nail peeling and even shattering or breaking below the free edge line because they effectively 'over-strengthen' their natural nails.
Most strengtheners use formaldehyde as the active ingredient for cross-linking. CND uses a patented ingredient called DMU instead of formaldehyde for a more controlled, hypoallergenic way to cross-link.