Many people wonder if vape juice have a shelf life, the answer is yes. There usually is an expiration date used best by on the bottle, however a correctly stored e-juice bottle should be perfectly fine to use for up to two years. The most important ingredients that determines an e-juice shelf life would be PG, VG and the nicotine oxidation. If a bottle of e-juice is stored correctly in the right environment it should be perfectly fine for up to two years.
Some factors that can shorten an e-juice shelf life would be direct sunlight, extreme heat. E-juice is best stored in room temperature away from heat, for example if an e-juice bottle is left in a hot car for a long time the juice gets darker and the PG and VG break down and loses its ability to recombine even when its placed back in room temperature. So, a change of the color of an e-juice bottle is one of the ways to tell if an e-juice had gone bad or not. Separation of e-juice usually happens when a bottle sits for a long time if you shake it well it should go back to its normal state, if it does not shake well then e-juice is expired. Another way you can tell is by the smell of an e-juice, if it goes bad the smell changes and it gives an odd scent. Last, if an e-juice bottle taste changes and starts giving a harsh or peppery taste that means that the nicotine has gone bad because a freshly baked nicotine do not usually have a taste.
In conclusion an e-juice does not usually have an expiration date and those that do it's usually an estimate, at the end of the day expired or bad e-juice causes no harm per-se but it is not a pleasant vaping experience!