From the Master Cleanse web site:
Misconception #8: The Master Cleanse is for everyone.
The truth: The Master Cleanse is great, but it's not for everyone. It
is probably the second most powerful (read that as the fastest) way to
detoxify the body. Only a water fast is more powerful. If a person has
a great deal of stored toxins, releasing them too quickly results in
detox symptoms, such as headaches, rashes, tiredness, irritability,
even vomiting. Those people should not do the Master Cleanse. They
should detoxify much more slowly with a better diet. Also, people
recovering from surgery need large amounts of protein to regenerate
tissue and so should not do the Master Cleanse until they have
completely recovered from surgery. Pregnant and nursing women should
not do it either, as there is a remote possibility that the toxins may
find their way into the infant. Although, I have never heard of such
actually occurring, but better safe than sorry.