When used alone, the nicotine and flavourings in e liquid produce no visible vapour. However, when propylene glycol (PG), a slightly sweet, colourless, viscous liquid is added to the e liquid recipe, a satisfying plume of vapour is the result.

When inhaled and exhaled, this vapour not only provides a similar physical sensation to that of inhaling tobacco smoke, it delivers the all-important fix of nicotine to the body via the lungs too.

BP or USP* approved propylene glycol is used in cosmetics, toiletries, food colourings, cake mixes, salad dressings, soft drinks and more. It's also used in the smoke/fog machines found in theatres and nightclubs.

Some people experience a sore throat, muscle pains and strong-smelling urine when first using electronic cigarettes. It's possible these symptoms are a result of vaping PG-based e liquid.

Because propylene glycol is a humectant (attracts moisture), it can dry your throat and leave it feeling sore. PG may also increase the body's production of lactic acid which could cause the muscle aches that some experience from time to time.

Most people find that drinking more fluids helps to flush lactic acid through the body more quickly which often resolves the problem. The sore throat should disappear on its own within a day or two, although increased fluid intake will help with this too.

If any of these symptoms persist or you begin to suffer from night sweats, diarrhoea or break out in a rash, you may have an allergy to propylene glycol.

We recommend you seek the advice of your GP and switch to a VG-based e liquid if you suffer from any of the symptoms above.

Luckily, if you're allergic to propylene glycol there is an excellent alternative – e liquid made with vegetable glycerine (ask your supplier for a VG-based e liquid). VG, also a virtually odourless viscous fluid although slightly thicker and sweeter than propylene glycol, does the same job with style. In fact, some e-smokers use VG-based e liquid for preference because they maintain it produces more vapour.

USP or BP approved vegetable glycerine is used in over 1500 applications, many of them additives to food and drink. Small bottles of glycerine can be found next to the food colouring and flavouring in the home baking section of most UK supermarkets.

Vegetable glycerine is also known as:

• Glycerin
• Glycerine
• VG
• Glycerol

*Only USP (United States Pharmacopeia) and BP (British Pharmacopeia) approved propylene glycol and vegetable glycerine should be considered food safe.