What? No, you feel better than you've imagined feeling and nod out and then stop breathing. It's faster with more but I guarantee no one ODing on morphine is feeling remotely bad.

Oh, I should also point out there are different kinds of opiates and opioids and your mileage will vary between, but I went with specifically morphine for a reason. Heroin is just morphine with greater bioavailibilty so you can just make that up with quantity as hospitals are wont to do for just normal anesthesia reasons.