not sure about the overdosin part, but you can generally smoke a couple little chunks on top of ur freshly packed bowl. My experience is with Red Rock Opium.
it's hard to OD on opium for the same reasons it's hard to OD smoking H. It hits you straight away, so once you've smoked enough to knock you out, you'll be knocked out, and unable to smoke more. If you snort or mainline stuff, you can take way too much and not know until it's too late. So I wouldn't worry about OD'ing.
Yea, you'll nod out when you've smoked enough so its not that easy to OD. Probably easier eating it, which I want to try at least once, but you'd need to eat a lot.
Don't try eating opium...why would you do that? Maybe tea... Red rock isn't opium... I dunno about noddin out. I'd assume you would, but I've smoked crazy amounts of opium in a single night, so you shouldn't worry about ODing unless you're smoking grams of the shit at a time.
