Detox can mean different things to different people. I am guessing you mean the Aajonus type where he tells you to keep eating things that he is convinced we all need to eat and can all metabolize, like milk or dairy in general or vege juice or eggs etc.
Then the obvious happens, Some people are fine with these things (me) but some people have zero tolerance and they get various versions of sick, ranging from discomfort to life threatening.
This is not to be confused with the person who really is full of toxicity from an awesomely bad diet and they experience a downloading of the crud they have been consuming which has lodged itself all over the body, causing a wide variety of chronic and acute illnesses.
I had a predictable, general, low grade lethargy that lasted quite awhile when I went from a vege diet (23 years) to eating raw meat. No doubt this was due to repopulation of the gut with different bacteria and a general resetting of digestive activities.
I was very healthy prior to the switch to raw primal , having virtually never had any significant illnesses. I got a few colds after starting the diet, but that was predictable, with an increase in fruit consumption which I slowed down and the colds went away.
The other type of detox is a planned/scheduled cleaning which I used to do under a supervised Ayurvedic regimen where I consumed a somewhat modified diet for a number of days in preparation and then on the last day I consumed a mixture of a couple of different types of laxative/purgatives that had complementary effects, combined with a hot bath with various methods/timings to keep things under control.
This produced a predictable "Holy sh*t"
as we used to call it.
Then we ate a prescribed diet for that day and maybe the next to repopulate the gut.
This is done at the end of each season and can be followed by an amazing process called Pancha Karma (seven cleansing actions to get rid of the karma. [actions or results of actions] in your body) This can include some incredible massages, steam baths, enemas, etc to completely flush toxicity out of you. This is done in odd day groups ( one, three or five etc) depending on what you have the time/money/inclination to do. The longer you do it, the more you clean toxicity out. You really feel calm during and afterwards. It's like resetting the clock and is best done in a retreat setting away from the hustle and bustle.
This is not the goofy stuff you see advertised at the health food store where some crank gets a great idea, takes a week-end course to learn how to pour God knows what up your wazoo and gives you a random massage and calls it a "herbal cleanse".