I've heard this claim so much but I can't possibly comprehend the basis of it.
Virtually nobody is out there overdosing and dying of caffeine (although it does happen). It's very addictive, yes, but if you deprived someone of their coffee for a month, they'll call you an asshole but they'll go about their normal life. With opiates, people are willing to live on the streets because they're more driven to put the drug over their personal well-being. I've known people who talked about having to empty their bowels with a spoon because they can't do it normally. Withdrawal is described as absolute hell and many people accept the possibility of overdose rather than the agony of going without it. People who built up a tolerance, quit using, then return often die because they took a little beyond their current tolerance. One major long term effect is that many people find it hard to find any joy in life as great as heroin.
It's not reefer madness. Thousands are dying and people are oftentimes knowingly sacrificing their lives for opiates. The argument that it's not bad when used responsibly and in small amounts is true of virtually every drug ever. In practice, many people aren't using them responsibly, and the nature of some drugs makes them even more apt to be abused than others.
People are desperate enough to see all the examples of people dying or otherwise destroying their lives for the drug but still deciding it's worth trying. The prohibition isn't the problem. The problem is people are struggling in life and opiates are providing an out. Making them more open and available will not help, and the oft-cited example of Portugal decriminalizing drugs doesn't mean every society should make opiates available. Drug overdoses recently increased in Portugal. [1]
[1] https://www.emcdda.europa.eu/countries/drug-reports/2019/por...