Diet pills, "tea-toxes" and appetite suppressant products are no quick fix, says Prof Stephen Powis, NHS medical director.
Products making this claim can have side-effects, including diarrhoea and heart issues, he warns.
Getting in shape safely takes time and requires eating sensibly, and exercise.
As the public gets ready for New Year's Eve, Prof Powis also warned of the dangers of "party drips" or quick fix hangover cures.
Last year, model Kendall Jenner was hospitalised following a bad reaction to a nutrient therapy IV drip, made up of saline solution, magnesium, calcium, B vitamins and vitamin C.
In extreme cases, regularly resorting to drips for hangover cures can cause nausea, liver damage, or death due to a toxic overdose of vitamin A.
Prof Powis said: "Miracle hangover cures and quick fixes simply don't exist, and anyone online who says they do is probably out to make a quick buck at your expense.
"New Year resolutions are a great time to make a change, but the reality is there's a slim chance of success with diet pills and detox teas - and people could end up doing more harm than good.
"Making New Year goals and shifting a few excess pounds after Christmas can be a good idea, but are much easier to maintain when done gradually and safely."
The NHS has tips to help people achieve their New Year health goals:
Lose weight: start the NHS 12-week weight loss plan
Quit smoking: download the NHS Smokefree app for advice and a free personal quit plan
Get active: boost your fitness with fun and practical ideas to help you get into shape, including Couch to 5K, Strength and Flex and the NHS Fitness Studio
Drink less alcohol: calculate your units, get tips on cutting down, track your drinking and find out where to get help and support
Eat more fruit and veg: whether you're cooking for a family or eating on the go, our tips and recipes can help you get your five a day
How to spot a fad diet
The British Dietetic Association (BDA) advises people to stay away from diets that promote a magic ingredient or product to solve your weight problem, without you having to change your lifestyle in any way.
Any diet that promises rapid weight loss of more than 2lbs (0.9kg) of body fat a week is also dubious.
Fad diets often promote eating only one type of food or avoiding whole food groups.
- Information and support: Eating disorders
- Beat's adult helpline can be reached on 0808 801 0677, or there is a dedicated Youthline for under-18s on 0808 801 0711.
And don't be fooled by celebrity endorsement - if it sounds too good to be true, the chances are it is.
The BDA says: "The best way to lose weight and keep it off is to make healthier choices, eat a nutritionally balanced and varied diet with appropriately sized portions, and be physically active."
The notion of "detox" is nonsense, says the BDA, although it is advisable to drink less alcohol and get more sleep, fresh air and exercise. The body has its own built-in system to detoxify and remove waste.
There are no pills or specific drinks, patches or lotions that can do a magic detoxifying job.