If you can take codeine you're not allergic to morphine. You may have some odd sensitivity to it, but you aren't allergic. What's it cause in you? Itchiness? heh.
I started with a hydrocodone prescription, and loved it. Erowid opened a world for me that I had never known. Heroin was unknown around here at that time, and even getting weed was hard. However, poppy pods were available and cheap back then, and I ordered thousands of them over a year and a half. I then got on methadone for an injury I had, told the doc I had been using illegally acquired painkillers because of the pain. Over time got propoxyphene, codeine, even fentanyl patches. Enjoyed it all immensely, but didn't realize the path I was heading on. Decided I wanted off methadone but couldn't do it, so I got on subs, back when no one really knew about them, I guess I was part of the first wave of sub users in the state, now I can probably think of a dozen people with scripts for it who I just met through random interaction, not at a meeting or anything. I don't know that many diabetics.
I used heroin a couple times back when I was on methadone, but didn't really enjoy it, probably because I was taking 80mg a day and I only tried 50mg at a try.
More recently, wanting an occasional alternative to subs, I'd get three or four 30 roxi's and use them on a day off. That was between 75 and 120 bucks to get high one day. With heroin 100mg will maybe get me high twice, if it's good, and I'll spend barely anything on it by comparison. For an occasional 'fun day' when I don't have to work, it's a good deal. For a daily thing? Bad idea. Dope is going to lead you to terrible places, places where pills would have, but now cash won't hold you back, and you can ruin yourself in horrible ways.
I am Hammilton, and I am an addict. I always thought my intelligence made me somehow less susceptible to addiction. I'm an idiot. Subs are saving my life, when I let them. Unfortunately I sometimes abuse them, too. Figures. I've always gotten high on subs, from my first dose to my last, I'll always feel decent from them. That encourages me to take more than I should. Only drug I've ever not really abused was kratom, and since it has no blocking effect, the compulsion to abuse better opioids a was too strong, and I ended up using more pills on kratom than anything.
This isn't a road you want to go down if you haven't. Heroin isn't glamorous, it's just a cheaper alternative.