Can drinkin' weight gainers cause any serious health harm (death)? i'm plannin' to take "optimum nutrition serious mass". thank you.

Not likely: The biggest issues that come about from using a supplement like this is weight gain. If you are not wise about your training and nutrition you can readily start to gain undesirable fat mass. Otherwise, unless you have diabetes, kidney issues, liver issues or significant GI absorption issues, it likely won't cause you any concerning adverse effects. Best of luck and stay healthy.