[Konstantinos]

Everything is fine with the example, it's just your fault that health is declared as global variable. Just change the name:

pawn Код:

public OnPlayerUpdate(playerid)
{
    new Float: fHealth;
    GetPlayerHealth(playerid, fHealth);
    if (fHealth < 50.0) SetPlayerHealth(playerid, 50.0);
    return 1;
}