I don't know if this helps (considering the sheer quantity of replies you have had) but I've had morphine in two different doses when my appendix was removed a few years ago.
The first injection was to remove the pain of an appendix that was inflamed. At first, the pain faded, which is where the job of a painkiller would normally stop. A few minutes later and the pain had gone and I felt extremely relaxed. A few minutes after that, I felt very comfortable and warm all over - like being extremely happy in a bath. The feeling continued until I fell asleep a little while later. The sleep was deep and restful, but I was awake six hours later due to the sound of a nebuliser in use by the man in the bed opposite me. At this time the pain was back and I had to call a nurse over. After she brought oral morphine, I found it a little easier to sleep after the pain went, though it didn't completely vanish as it did with the injections.
The second time was just after the appendix was removed. I was extremely sluggish after the anesthetic, but could communicate how much the just-sewn-up wound was with a lot of effort that came out as grunting and the ability to just about move my fingers. That time, I was given an injection of morphine, left for a few minutes for it to take effect, and then asked by a nurse how it had managed the pain. Surgical wound > inflamed appendix, so after the second injection the pain was still pretty bad. After the third injection and a warning that any more and my heart would stop, the pain had gone from a 4/10 to a happy 0/10.
The increased dosage meant that euphoria set in. To set the scene, I was not a generally happy or accepting person (I'm really nice now, though). There was a guy in the ward whose bed was opposite me who believed everything he read in the British tabloids, and while I couldn't blame the other guy who couldn't breathe unassisted I still wanted to destroy his machine because it kept me up for about an hour last night.
They were now my best friends in the whole universe.
By the time they wheeled by back to the ward, I had turned from a seething bag of hate into friendliness incarnate. My senses became fuzzy and the warmth was very, very relaxing. I remember propping myself up onto my elbows, being wheeled into the room and asking a very happy "alright, mate?" to the man opposite me who looked over his copy of The Sun and asked the nurse what they had done to me. She said that I would be asleep in a few minutes, but I felt full of energy though couldn't really move my limbs to expend any and the colours in the room seemed more vibrant. While I was moved around, I asked mechanical lungs dude how he was and he couldn't do anything except look at me while I gave him a huge thumbs up for no reason at all. As this is England, I was committing massive social transgressions by being loud, friendly and happy. I think I shouted thanks to the nurse and porter as they left me, and then I tried to sleep. Sleep didn't come for ten minutes, but it lasted from around midday to 4PM where I was given more of the happy juice.
At 6PM, I expected visitors.
So my friend James is there trying to make nice while I'm wondering why I can't piss (anesthetic) and walking around offering to get nurses crisps from the vending machine because they're my new best chums. James laughed at me and went home, wherein I then 'entertained' a friend and her mother. I say entertained because I was very hot due to the heating in the hospital being cranked as high as I was. The effect? I'm laying on a bed in my underwear talking to two women while absent-mindedly fondling my genitals that are getting a little ripe. They pointed out what I was doing, so I stopped. Then I kept doing it without realising. I didn't care, and while I knew they didn't want to see what I was doing, it really didn't bother me that much. They left and I tried to read, though I couldn't focus on what I was reading so I tried listening to music, though I couldn't really follow it as my mind kept skipping around. So while I could hold a conversation, I couldn't really concentrate on anything.
Next morning, I found I still didn't have an appetite despite the nurse telling me I should eat. I did eat and I was sick, though it wasn't a 'hmm, something doesn't feel right, I might puke in a moment' kind of sick, it was a 'hey man, this place is really hot and BLARF' kind of sick that was still really hard.