//This will work for a very specific landanum, only when you use that specific landanum.
void OnStart()
{
    AddTimer("Landanum", 1.0f, "CheckLandanum");
}

void CheckLandanum(string &in asTimer)
{
    if(HasItem("potion_health_1"))
    {
        SetLocalVarInt("Var1", 1);
        AddTimer("Landanum", 1.0f, "CheckLandanum");
    }
    else
    {
        if(GetLocalVarInt("Var1") == 1)
        {
            OnPlayerUseLandanum();
        }
        else AddTimer("Landanum", 1.0f, "CheckLandanum");
    }
}
void OnPlayerUseLandanum()
{

}