Initiation of use:
So I think prohibition really affected my decision to try heroin in the first place. A big part of this was that the drug education I received was mostly the "all drugs are bad and will rapidly destroy your life" type. We were lied to about drugs a lot, and illegal drugs were all sort of lumped together as "bad" and "just say no". I had tried things like cannabis and LSD and found that the authority figures were lying about those as while as MANY other aspects of drug use, like the idea that teens only try drugs due to "peer pressure" or the effects of the drugs themselves) and so I assumed everything I'd been told about heroin was a lie too. I think I may have also kind of been attracted to the illegality and stigma surrounding it, it made me more curious and I was an unconventional teen who felt drug laws mainly existed to control people. I also mistakenly thought (like many) that I would be able to control my use and not get addicted, in part because of societal beliefs which were especially prevalent at the time (due to the "War on Drugs" IMO) that drug addicts are just weak, bad people who have no self-control or choose to be addicted (this was said a lot, that "addiction is a choice"). Not that I believed judgments about drug addicts, but I did think I could deliberately avoid addiction. I also didn't understand the difference between mental addiction and physiological dependence and thought you could only get addicted if you used it every day, so I would just not use it often enough to get addicted, in fact at first I think I thought I would just try it once (these are all still common misconceptions for many people who start using heroin). After trying it once and liking it, I figured gee I'm not instantly addicted like I was told I would be, so I'm probably right that I can avoid addiction and it's probably fine to use heroin once in a while. I didn't know addiction built slowly over time. Easy access to
truthful information about drugs would have made a big difference I feel.
Eventually I started using heroin to cope with emotional and physical pain (actually I'm sure that's what I was doing unknowingly from the beginning, even though curiosity, rebellion and the belief that there was no real harm in trying it were the main
conscious reasons for that initial use). Somewhere along the line I just wasn't able to make myself go very long without using it anymore. Doctors and family weren't helping me and I'd found something effective. Doctors didn't take me seriously or help me, they just treated me like I was making up my problems or that kids/teens just want drugs (an indirect consequence of prohibition I feel), even though I never specifically asked doctors for drugs and non-drug options were not suggested by them.
Obviously merely making heroin legal and not changing anything else at all would not have reduced most of these problems, but I think that making changes to laws, education, health care, societal beliefs, access to support for mental illness or abusive situations, etc could greatly reduce them. If I lived in a different society and had different drug education and support I probably would never have tried heroin in the first place, even if it was completely legal and easily accessible.
Harm from drug use:
It was difficult to access harm reduction materials and there was little education about harm reduction. Being the type of person that I am I actively sought out this knowledge, but advice was often limited to very basic things like "don't share needles with others". I got books from the library on heroin before even using it but they were sometimes wildly inaccurate, like one said that heroin has no adverse impacts on the human body or brain and the only risk from it was overdose, whereas others were overly dramatic in the opposite direction - guess which one I believed?

There was a small needle exchange operation in town with limited hours and one could sometimes get syringes from certain pharmacies, but neither were convenient and the pharmacies would often treat me like crap or refuse to sell them to me. For quite some time I didn't know there were any risks from reusing your own needles or injecting in the same spot over and over and I totally destroyed my veins that way. There was certainly no access to things like micron filters or reagent testing kits. Yes harm reduction could potentially be better promoted while the drugs themselves remained illegal, and it's certainly easier to access HR than it was back when I first started using, but I think there would have to be some major changes in laws and societal views on drug use for HR under prohibition to be as effective as possible.
Overdoses:
I believe that if drug laws were different I may not have overdosed. One overdose, as I mentioned, was from using street heroin in the same quantity as usual - if it were regulated/legal/standardized in some way I would not have ended up with a batch that was substantially more pure than the last. I was lucky that I had a friend there who was able to call an ambulance. Another overdose was from cocaine that was sold to me as heroin. I had a very high tolerance to heroin at the time and no tolerance to cocaine and overdosed on the cocaine. Obviously that wouldn't happen if people could be assured of what drug they were getting. I have also had adverse effects (unpleasant side effects, vein damage, etc) from getting street heroin that contained various impurities and cuts - again that wouldn't have happened if I could have been assured of getting pure heroin.
Costs:
This is a tough one, because I don't think that free unlimited opioids for first-time users is a good idea, nor do I think that addicts should have to pay outrageous amounts of money to maintain their addiction, so an ideal regulation would have to be somewhere in the middle and carefully and properly executed. I am absolutely in favour of
at minimum decriminalization of buying and possessing drugs for personal use, but I think an ideal situation would be some form of regulated legalization, because decriminalization would only reduce
some of the harms associated with using drugs - we would still have high prices, drugs that are impure and unreliable, crime to obtain drugs, etc. The high financial cost of my heroin addiction was a huge impact on my life. I couldn't function without it, so, until I eventually became a high-end escort, my life pretty much revolved around getting money for drugs and I put myself in all sorts of sketchy situations due to that. Let me know if you'd like specifics.
General problems due to illegality:
Having unreliable sources, times where my supplier would disappear and I wouldn't be able to function at all because I was so sick and so depressed, having to spend a lot of my time and effort obtaining drugs, being in all sorts of dangerous situations, being robbed/ripped off, being around people who were unhealthy for me, fear of arrest, etc. I had close friends who went to prison because of their drug use, for example someone selling drugs in order to pay for their habit and someone who was stealing to pay for their habit. Prison did NOT help them at all.
Recovery:
I feel my likelihood of quitting heroin and quitting opioids completely was greatly impaired by prohibition and everything that goes with the War on Drugs, societal beliefs, lack of truthful education about drugs, etc. When I first decided I wanted to quit heroin my only options were unmedicated short-term detox (with a waiting list), Alcoholics/Narcotics Anonymous (the goal being abstinence by going to meetings and using the 12 steps and people who were on drugs were not welcome at meetings), or methadone maintenance (with a waiting list). Quitting "cold turkey" and NA/AA did not work for me, so I went with methadone. I was told that methadone would completely eliminate all withdrawal symptoms and all cravings for heroin, had no adverse physical or mental health consequences of its own, and was much easier to quit than heroin (stuff like "you can easily taper off it with no withdrawal symptoms whenever you decide to, so there should be no rush trying to get off it," and "even if you were to stop methadone suddenly the withdrawal symptoms are much milder than heroin's"), which are all completely untrue. The societal belief was that methadone was "just legal heroin" and there were a lot of judgements about it. I truly believe that if I could have actually had legal prescribed heroin or morphine that would have been much better for me and that I would have quit sooner. Many of the problems I had with the methadone program was could definitely have been improved by changes to laws/regulations and education. Access to other tools for recovery (including some illegal forms that I have used) could definitely be improved upon.
*I am getting tired of typing so I'm going to stop here for now but I can elaborate more on the recovery section and what changes I would make to how things currently are later because there is
a lot to it.