What is it?

Casein is the protein of the solid portion of milk, which is obtained after separating it from the liquid part (whey) during the production of cheese. It amounts to about 80% of cow's milk and 35% of human milk. When casein mixes with the acid elements in the stomach it creates a jelly substance that slows down digestion and leads to a slow release of amino acids in the bloodstream. This is why it is often referred to as a "slow protein". A characteristic that distinguishes casein is its resistance to heat: when heated up it maintains its chemical structure and is not denatured. The biological value of caseins is 77: lower than whey proteins. Its amino acid composition is also different than whey proteins, being in fact very rich in glutamine and proline and poorer in branched amino acids than whey proteins. 

Just after being taken, casein creates a gel in the stomach, therefore the intake must be separated in time from the intake of any pharmaceutical drugs, as it could reduce absorption and therefore effectiveness. 
Caseins can synergize with pressure control drugs, enhancing their effect. Indeed, casein peptides may even have a slightly hypertensive effect (Fekete et al, Nutrients, 2015).

+Watt advices

How to use: even if they have a lower biological value than whey proteins, caseins effectively contrast the muscle catabolism thanks to their slow absorption speed and the high content of glutamine. This is why they are excellent to maintain muscle mass. It is recommended to take them in the evening, before going to bed. The high content of proline, a key component of collagen and therefore also of cartilage, makes caseins useful also for recovery after sport injuries, especially after tendinitis or bursitis.