Speaking of legal opioids, or what used to be legal, unsure now, but it disappeared from from the EU at least.
Wiki
"Tianeptine, sold under the brand names Stablon, Tatinol, and Coaxil among others, is an atypical tricyclic antidepressant which is used mainly in the treatment of major depressive disorder, although it may also be used to treat anxiety, asthma, and irritable bowel syndrome."It was an antidepressant in like 12.5mg pills. What they apparently didn't even know was that it was an agonist at opioid receptors which could really be felt if you raised the dose. "Gas station heroin," they used to call it in the US. It was pretty great, only used it pretty briefly but it was a very nice mood brightener. Didn't feel any withdrawals when stopping, but they must be present if used long enough, it was just too good.
Regarding Kratom, for a few years I played with the 2-3 gram doses and couldn't figure out why people were raving about it, I could feel a little something but not enough to get excited like others on the net. Then years late I decided to try 10 grams, now I felt it and it was a first dose, was pretty damn euphoric combined with a little something else. Then maybe six months ago I was in great pains and ordered 400 grams and tried a 20 gram dose, some days 2x20 grams so it was a very short cycle. In every case I used it, even for a few months straight but at lower dose, I only felt mild withdrawals for a few days. Restless legs for 3 days, but didn't really cause insomnia which is what I'd gauge bad withdrawals with, maybe just 2 days of not sleeping quite as well as usual. But I read on some forums, like professionalmuscle, where some say it's every bit as bad as Oxy

Some of them buy the straight 7-mitragynine synthetic so it's probably much worse. For years I figured that obviously some sellers spiked their Kratom powder with 7-OH so buyers would think their Kratom was "better." Otherwise I figure it's all the same shit.
"How strong is 7 mitragynine?
Pain Relief: 7-hydroxymitragynine is known for its potent analgesic effects, relieving pain by acting on the mu-opioid receptors. Research findings suggest that 7-OH is approximately 13 times more potent than morphine."