Nicotine gets metabolised into several compounds within the human body which are carcinogenic, even if pure nicotine in itself isn't.

Cancer risk is more complex than just carcinogens. Nicotine is known to promote the growth of existing cancer cells, and in multiple ways. A big thing with cancer that not many people are aware of is that we all have cancer cells, and get new cancer cells all the time — but that the human immune system is normally effective at detecting and killing them before they have multiplied too much. Old cancer mutations can lay dormant or kept in check for many years, but if promoted and/or the immune system gets stressed or suppressed, they'd grow and you'd "get cancer".

Different types of E-juice also contain additives for flavour, and we still don't yet know the long-term effects of some of those — when ingested as vape — which is a different to being swallowed. And by long-term, I mean 20 years or more, which in some cases is the time a cancer cell can take from formation to detectable tumour.