I would guess that this varies a lot by country, gender, sexual orientation, and the status of the production company.
I'm going to hazard a guess that most male porn actors use nothing on set besides erection aids, and that any producers and directors that care about company profits warn their male stars not to touch anything, lest they be unable to rise to the occasion, or get into a mood that makes the actress and crew not like working with them. Think about it -- there are so many qualified men willing to do porn, that producers of any repute can probably more than afford to thin the appicant pool to only those male actors who are able to give a stellar, uninhibited performance every time stone cold sober. Also, don't the actresses get to choose who they want to work with? I bet a lot of them will only work with a man onset that they can trust is sober. If I were a woman about to get fucked in all kinds of uncomfortable positions for a few hours by a total stranger, I know I'd want him to be sober, for the sake of my own safety and comfort. Last I heard, straight male actors were fairly disposable in this industry.
As for female performers, my guess is that there's a 'don't ask don't tell' approach to drug use. Maybe they get told something by the director under his breath. Maybe they get made to sign something attesting that they were sober at the time, for legal purposes. But since they're the ones that bring in all the money in the industry, they're kind of like rockstars, and I bet a blind eye gets turned to anything they do offset, except catching an STD.
But this is all speculation, based on the few facts I know about how this industry is driven.
Also consider this. There are many eager consumers of porn who are not consumers of drugs at all. Any reputable production company would not want to risk losing money or reputation, because some gherkin jerker noticed the actors and actresses looked wasted and raised hell on some blog.
Just because a form of recreation or entertainment breaks one social taboo (sex in public between strangers for others' viewing pleasure, in the case of porn), does NOT mean fans or participants are open to seeing ANOTHER social taboo broken. In fact, the LACK of an attitude of 'We're already on this side of the fence, so now anything goes!' probably goes a long way to making fans of socially taboo forms of fun feel more comfortable and less guilty. For example, although I'm not into these scenes at all, I've heard many swingers parties ban drugs strictly, and many BDSM groups forbid public acts that are explicitly sexual, even though pain is perfectly fine. I was also not an old school raver, but I hear that sexual and lecherous behavior used to be very much frowned upon in the early rave scene, no matter how awash in drugs it was. I would bet that most fight clubs are very sober places too.