If you have an actual reliable, regulated source then it's possible to use it responsibly then. As long as you have the ability to verify the purity and safety of your doses and the ability to reliably regulate your doses then you can use heroin responsibly.

If you can't get heroin from a reliable source, and if you can't verify the safety of that source, you can't use it responsibly.. because it's not responsible to use heroin without a source you're 100% sure won't be spiked or diluted with other harmful things. Same goes for a lot of stuff.

You could dose yourself responsibly, but the issue lies with the source and risk of other harmful things being mixed with it.

I'd say the same for mdma, even as a twice-a-year-for-a-decade user.. even if I dose myself responsibly I'm still taking a risk because ultimately I don't actually know what I'm putting in my body.. I trust the dealer to acquire a pure product. Unless that dealer is a doctor acquiring the product from a legit lab source under regulation.. it's technically irresponsible to put that in my body.

Street heroin you never know what it is, can't be used responsibly. Unless you can do a lab test to verify the purity.