Ianto doesn't mean to just be grabbing things out of John's hands. It's just a culmination of things, he's crawling out of his skin, anxious, uncomfortable, and way too in his own head, and honestly if there is a chance that this will help him calm down then he's all for it.

He does at least mutter a word of thanks as he takes the book of matches from the other man, lighting one to light the little roller up. "Here goes nothing then," Ianto says, glancing to the other man a little anxiously, because he's well aware that this is uncharted waters and probably a terrible idea, before he puts it to his lips and inhales, letting the breath out in a slow cloud of smoke.

It tastes... Pretty much how one might expect, having seen what it was before it was rolled up. Ianto brings the little roller to his lips again, inhaling again because it's pretty much go big or go home at this point, and tries to get a handle on how he's feeling. If it's making him feel anything. He thinks there might be something there. Maybe he should just keep smoking it to find out.