Possible side effects:

Fasters must take care to maintain their intake of vitamins and nutrients. However, there are no specific side effects associated exclusively with juice fasting, though certain medical conditions such as diabetes may be aggravated by excessive intake of certain juices.

It's also possible for a juice such as grapefruit to interact badly with certain prescription drugs.

Additionally, the juices from highly acidic fruits such as tomato and citrus juices can conceivably upset the body's natural acid-base (pH) balance.