new Dying[MAX_PLAYERS]; //Will detect whether they are dying or not

public OnPlayerConnect(playerid)
{
    Dying[playerid] = 0; //Player is not currently dying
    return 1;
}

public OnPlayerTakeDamage(playerid, issuerid, Float:amount, weaponid) //Called when a player takes damage
{
    new Float:hp;
    GetPlayerHealth(playerid, hp); //Get the player's health
    if(hp < 40.0) //If their health is lower than 40
    {
        if(Dying[playerid] == 0) //If they are not already dying (We don't want to set a timer twice)
        {
            SetTimerEx("DeathTimer", 3000, true, "i", playerid); //Set a timer every 3 seconds to go off (repeating)
            Dying[playerid] = 1; //They are now dying
            SendClientMessage(playerid, 0xFF0000FF, "You are injured and losing health slowly."); //Change this message however you want, or remove it, the choice is yours
        }
    }
    return 1;
}

public OnPlayerDeath(playerid, killerid, reason) //Called when a player dies
{
    if(Dying[playerid] == 1)
    {
        KillTimer(DeathTimer(playerid));
        Dying[playerid] = 0;
    }
    return 1;
}

forward DeathTimer(playerid);
public DeathTimer(playerid) //Our timer that will activate every 3 seconds
{
    if(Dying[playerid] == 1) //If they are dying
    {
        new Float:hp;
        GetPlayerHealth(playerid, hp);
        if(hp < 40) //If health is still below 40 *Bug prevention*
        {
            SetPlayerHealth(playerid, hp - 1); //Take 1 off their health
        }
        else
        {
            Dying[playerid] = 0; //Health is not below 40, they are no longer dying
            KillTimer(DeathTimer(playerid)); //They are no longer dying
        }
    }
    else if(Dying[playerid] == 0)
    {
        KillTimer(DeathTimer(playerid)); //If they are not dying, they no longer need the timer, so kill it to prevent bugging.
    }
    return 1;
}