If you were to create a new drug and have it approved by the FDA, you would need to start first with lab tests at the cellular level, then proceed to animal models, then to pilot studies, then to several phases of clinical trials to show that the drug has the effect you are seeking and that not too many people are killed by it in the process. After you finally get the stamp of approval to sell it, you have to do post-marketing surveillance to follow what goes on when the drug is in wide-spread use. Clinical trials often exclude people who are already taking a different medication, who are being treated for other diseases, who have heart problems, etc., etc.
What scientists see happening at the cellular level doesn't always translate into what happens in a human body. Chinese hamster ovary cells might not have the same naturally-occurring cancer-fighting substances present in humans. Chinese hamsters might not eat all of the same foods that cancer-free humans eat. There are just too many variables in play to make a definite decision based on cellular or animal studies. The acid test is the post-market surveillance.
If PVs were to be considered as a medication, they did things backwards to some extent. They have already been out on the market since 2004 world-wide. In general what happens when a smoker switches to e-cigarettes is that their health improves--especially if they smoked long enough to be experiencing some of the usual smoking-related conditions such as coughing, wheezing, etc.
The best information researchers have at present on long-term use of a nicotine product is all the research conducted in Sweden on smokers who switched to snus. In fact, this research was the basis of the presentation that Dr. Neal Benowitz gave to the FDA when they held a public hearing on whether to approve long-term us of NRTs. Supposedly NRT stands for "nicotine replacement therapy", but we who were on the wheel of misfortune have decided that the R really stands for "reduction."
But I digress. Here is a link to the Benowitz presentation.
http://www.fda.gov/downloads/Drugs/NewsEvents/UCM232147.pdf
Here is what Benowitz said about cancer and smokeless tobacco (ST) use, "The lack of increase in common cancers in lifelong ST users indicates that nicotine is not a general cancer promoter." And regarding cardiovascular disease (CVD) he wrote, "Meta-analyses showing increased risk of MI and stroke in ST users are heavily weighted by CPS-I and CPS-II, which are older US studies with many methodologic problems. More recent Swedish studies and an NHANES study indicate minimal if any increased risk of CVD with ST."
There was one caution: "Nicotine likely has adverse effects on reproduction, including increasing the risk of pre-eclampsia and preterm birth."
Of course, this is based on the nicotine intake and TSNAs present in snus. People tend to take in less nicotine from vaping than they do from smoking, and the TSNA levels are much, much smaller.
Snus is processed in such a way to reduce the content of Tobacco-Specific Nitrosamines, thought to be the cancer-causing chemicals in cigarettes. To give you an idea of the range of TSNA content (calculated daily exposure with average use):
one each, 4 mg., Nicotine patch - 8 nanograms
1.5 gram, 16 mg e-cigarette liquid - 12 nanograms
20 pieces, Ariva dissolvable orbs -1,045 nanograms
8 grams of Swedish Snus (General) - 16,000 nanograms
1 pack of Marlboro full flavor -126,000 nanograms
So if 16,000 ng per day of TSNAs from General snus doesn't cause any type of cancer, it seems pretty improbable that 8 ng per day (or even 24 if you vape a lot) would cause cancer.
