When fruits and vegetables are squeezed or used raw, bacteria from the products can end up in the juice or cider. Unless the product or juice has been pasteurized or otherwise treated to destroy any harmful bacteria, the juice could be contaminated. Juice concentrates come in several varieties that tend to vary in quality and fruit content. To choose the healthiest option, choose 100% fruit concentrate.
Juice alone is fine, but just drinking juice during a cleanse is when things get problematic, as there's no fiber that crosses your gastrointestinal tract to keep you regular. The idea behind a juice cleanse is that drinking juice as the only source of food allows the body to rid itself of “toxins”. That doesn't mean that drinking juice is bad and that you should never do it. Of course, we're not here to judge your choices about the things you eat and drink, and if you want a juice, drink that juice and enjoy it.
Drinking juice for breakfast or doing a juice cleanse may seem like a good idea, but this habit can actually be harmful to some people.