Err...all nitrites are potential carcinogens. Why?
Because they react with secondary amines in the body to produce nitrous amines - many of which are very powerful carcinogens. Check out some of my older posts for specific references.
Nitrosamines are also produced in small amounts naturally in the body due to enzymatic actions on nitrates, which are in many vegetables. Nitrites are also used as preservatives in processed meats (incl bacon) and beer, although antioxidants are now added to these foodstuffs which limit nitrosamine formation. Such was the concern in the 70's, that until it was found that nitrosamine formation was inhibited by Vit C, serious thought was given to removing bacon altogether from the US market. Sodium nitrite and salt greatly inhibit growth of botulism, and suitable replacements just don't seem to exist. NaNO2 also gives bacon and ham it's characteristic colour
Taking large amounts of a nitrite inhalant means potential for nitrosamine formation is far higher, particularly when taken with/after drugs containing a secondary amine group. MDMA is of course, a secondary amine.
To give you an idea of just how nasty nitrous amines are, think about one of the risks associated with smoking - pancreatic cancer. Smoking increases the risks of contracting pancreatic cancer, most likely because of the amounts of nitrous amines produced in cigarette smoke. Pancreatic cancer is supposed to be rare although I've known 5 people in the last 3 years that have died from it. It is almost always fatal. What's most alarming is the risk of contracting pancreatic cancer after giving up smoking. The risk ( nearly double that of someone who has never smoked) persists for a decade or more after stopping. While it is yet to conclusively proven, it is generally believed that it's due to nitrosamines.
So, while occasional use probably doesn't present too much of a problem in this regard, it's worth noting these properties and realising nitrites are not desirable chemicals to be consuming if you're concerned about long term health.
Micks warnings on alcohol and nitrites should be noted. Also the warnings on never taking Viagra with nitrites should be heeded, as this combo can cause extreme hypotension.
Personally, from someone who once got amyl in little vials (used as a cyanide antidote back in the 70s), I hate the stuff. It makes my head feel and look like a pumpkin. IMHO Alkyl nitrites (isobutyl is generally what's sold in shops) should be used for what they're best suited for - as solvents or reactants in syntheses.