What are they? Also known as thermogenics, these are blends of herbs and stimulants that slightly increase your body temperature, which can help you burn more calories during exercise. Ephedrine, a synthetic version of the Chinese herb ephedra, used to be a key ingredient in these, but it’s now only available on prescription in the UK due to its harmful side effects and addictive qualities.

What do they do? Some simply burn calories as heat. Others also claim to stimulate the release of adrenaline, increase your metabolic rate or act as appetite suppressants. The evidence for them working is limited, however. ‘A careful calorie intake and exercise are likely to produce better weight-loss results in the long term,’ says Anita Bean.

Who should take them? ‘Fat burners raise cortisol – a stress hormone – so if you suffer from anxiety it could make things worse.’ says Gregg Marsh. ‘If you think you need them consult your doctor first.’

How much should I take? Follow the instructions on the bottle, but be careful with long-term use. ‘Take them on a rotation cycle of 14 days on 14 days off for only two cycles every eight weeks,’ advises Marsh.

When should I take them? Most contain caffeine and will make you jittery, so the morning’s probably best. ‘Never take fat burners after 2pm because they affect sleep patterns,’ says Marsh. Other than that, go with the recommendation on the bottle.  

Do they have any side effects? ‘Taking very high doses of ephedrine can have serious effects, including palpitations, anxiety, insomnia, vomiting and dizziness,’ says Bean. ‘While herbal alternatives are generally safer, you may get side effects with high doses – some can raise blood pressure or cause heart disturbances.’

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