Then it might be as suggested by others, it is not heroin but some other opiate analogue to heroin, even in the real world oxycontin was sold as a "non addictive" alternative to other opiates, and yet it was highly addictive, the company which marketed it as non-addictive were fully aware of that reality and sold it to the public, and lobbied (bribed) politicians to make it easier for them to sell it to the public, Tommy wanted to try something, and Quinn provided it, if her telling him that it's not really addictive is enough to push him over the edge, then it is something that he already wants to believe
And that is exactly why Quinn has the line to Tommy when he says that he's "out", it was something like "so this is the paradigm shift in our relation, you're going to turn up like an addict looking for your next fix from now on?", Quinn is selling an opiate to Tommy, regardless of how unlikely you believe it to be, and as I've stated previously, she's not a well established dealer, she only recently started selling harder stuff, and she has only been selling at B&R for less than a year since she started attending B&R roughly one year previous to the events of BADIK.
When you're starting out as a drug dealer, or you're starting out slinging some new stuff, you're going to have a more limited clientele, and she is actively looking to grow the number of people she's selling to, Tommy even convinces her that he can bring more people in to use the new opiates, he says that they'll use the DIK mansion as their trap house, and that he's willing to be the scape-goat for her if shit hits the fan, those are literally conversations in the game.
