Juice cleanses and extreme fasting are often touted as crucial elements of a healthy lifestyle.
Unfortunately, those whose detoxification organs are already overburdened (due to things like mold exposure or an excess of viral debris after a long illness) do not do well with long-term cleanses and fasting because they naturally detoxify less well than others.
In other words, those who do really well with juice cleanses or fasting protocols usually do not have high levels of stored 'toxins' to begin with.
While techniques like intermittent fasting (where eating is confined within a 6, 8 or 12 hour window) can be successfully used to improve health, many people get into trouble doing water fasts that last for days and juice fasts that last for weeks or months. 
I am a certified Functional Medicine doctor who has specialized in chronic and complex illness for over a decade. Lately, many patients have arrived in my care as a result of a fast or cleanse that seriously compromised their health.
Why Cleanses and Fasts May Impair Your Health
By 'forcing' detoxification with prolonged fasts and cleanses, these patients can put a strain on their overall health, and they can develop new symptoms that last for months or years.
This worsening of symptoms is can be due to these patients having specific genetic variants affecting methylation and glutathione-related detoxification such as MTHFR, MRT, PEMT, GSTP1 and others.  
These genes cause their bodies to have to work harder to maintain healthy detoxification pathways.
A worsening of symptoms may also be due to long periods of exposure to environmental or chemical pollutants. Mold exposure either in the distant past or present can also contribute to a significantly decreased detoxification capacity.
Detoxification pathways are the routes in which debris travels to be escorted out of your body through bodily waste like urine, mucus, sweat, and bowel movements.
When cleanses or fasts are done by these patients, it forces their liver, kidneys, and adrenal glands to work very hard, very quickly - squeezing large amounts of waste products into their bloodstream.
These waste products are dumped too fast to be efficiently escorted out of their body. It clogs their already strained detoxification pathways and creates additional (potentially long-term) symptoms.
What is Herxheimer's Reaction - And What Isn't
Sometimes a severe reaction to detoxification is explained away as Herxheimer's reaction - where your body is rapidly clearing up an infection, and the 'die-off' causes viral debris to clog your detoxification channels leading to mild fever, nausea, and rapid heart rate.
Patients are often urged to 'push through' these symptoms and assured that they will feel better soon.
There are ways to support your body through a genuine Herxheimer's reaction (like when you are treating a viral infection), but I never recommend maintaining a detoxification therapy at the same level when you have prolonged severe symptoms.
Those symptoms are your body's way of telling you to stop.
Cultural Differences in Treatment Techniques
My American patients are the ones most often to go overboard with detoxification protocols; they believe that their goals are best achieved through the most difficult intervention, and pain and discomfort are signs that it is 'working.'
However, if you keep pushing yourself past the point when you have severe symptoms, you can do lasting damage and make those symptoms permanent.
I had a patient who came to me after a month-long juice cleanse - it had impaired her gastrointestinal tract, her stomach function and led to months of headaches, fatigue, anxiety and depression.
Faster is not better. More is not better. If you push your body too hard in this way, your recovery will take longer. You most likely embarked on a water cleanse or a long-term juice cleanse because you desperately want to feel better. For this to be accomplished, a strategic approach is always the best.
Your doctor should know what your body can handle during treatment and advise against extremes if your body already has difficulty detoxifying. This information is gathered by having patient appointments that include a thorough patient history and, if necessary, specialized testing.
Again, certain kinds of fasting can be very beneficial but fasting protocols should be started at the optimal time so that it does you the most good. If a fasting protocol is making you feel much worse, something is off and the parameters of your diet should be evaluated to make sure your body can handle detoxification at that time.
I treat patients with many underlying conditions and 'mysterious' symptoms who have been labeled as 'hard to treat' by other doctors. I help these patients by determining how much they can handle at any given time.
Rather than adding extreme diets or strict regimens, I remove the biochemical and psychological roadblocks by healing things like underlying viral infections, gastrointestinal dysbiosis, food allergies, and vitamin deficiencies.
I never do blanket protocols for everyone. They simply don't work for complex patients or people who have had symptoms for a long time.
The Shortcomings of Assembling A DIY Treatment Plan
Just as the same workout regimen does not work for everyone, the same medical treatments will not either.
The abundance of medical information online can cause the most well-informed patients to pick and choose from research-backed interventions - like supplements and diets - to form a DIY treatment plan for themselves. This can work for some people, but often, this is how they arrive in my care.
Just because medical research backs an intervention does not mean it is the best approach for you. Medical research is often hard to replicate, and many studies have misleading titles, such as those touting a particular detoxification supplement but, in reality, have only been tested on a tiny population or used in mice experiments.
Academic research also incentivizes 'useless' research that does not benefit patient populations.  As many other doctors would agree, some treatments are great on paper but simply don't work on their patients.
Functional Medicine To Help You Support Detoxification
Make sure you find a practitioner trained in Functional Medicine who can read and administer the latest testing. If a medical practice has blanket 6-month or 12-month 'protocols' that stay the same and do not adapt to the patient's shifting needs, it may not be the best for you.
As you heal, it is like peeling back an onion layer by layer. Medical treatment plans should shift over time to accommodate your new medical reality.
Just as you don't train for a marathon by running 100m dash sprints every day, health is not universally restored by everyone doing water fasts or juice cleanses.
Listen to your body - if it is telling you a diet or detox regimen is not suitable for you, then get some medical advice on how to proceed.
I am currently accepting new patients and offer online consultations worldwide.
Disclaimer: If you think that you may have a medical emergency, call your doctor or 911 immediately. No action or inaction should be taken based solely on the contents of this information. Nor should you ever delay seeking medical advice or treatment due to the information contained on this Website. The information on this Website is not intended to diagnose, treat, cure, or prevent any disease or medical condition. The information discussed is not intended to replace the advice of your healthcare provider. Reliance on the information provided by this Website, Dr. Brian Lum, or Functional Healthcare Institute is solely at your own risk.
 The Institute for Functional Medicine. "Fasting and Chronic Illness." https://www.ifm.org/news-insights/fasting-chronic-illness/
 Cui J, Li G, Yin J, Li L, Tan Y, Wei H, Liu B, Deng L, Tang J, Chen Y, Yi L. GSTP1 and cancer: Expression, methylation, polymorphisms and signaling (Review). Int J Oncol. 2020 Apr;56(4):867-878. doi: 10.3892/ijo.2020.4979. Epub 2020 Feb 10. PMID: 32319549.
 Li WX, Dai SX, Zheng JJ, Liu JQ, Huang JF. Homocysteine Metabolism Gene Polymorphisms (MTHFR C677T, MTHFR A1298C, MTR A2756G and MTRR A66G) Jointly Elevate the Risk of Folate Deficiency. Nutrients. 2015 Aug 10;7(8):6670-87. doi: 10.3390/nu7085303. PMID: 26266420; PMCID: PMC4555142.
 Collier, Roger. 2016. “Is Most Clinical Research Useless?” Canadian Medical Association Journal (CMAJ) 188 (11): 790–91. https://doi.org/10.1503/cmaj.109-5296.