I used Organic 100% Lemon Juice "not from concentrate". I used 2 tbsp of the Organic Lemon juice, and it didn't give me any ill effects at all. Just make sure that when you get it, that the Ingredients say "Organic Lemon Juice" and nothing else. I found it at my grocery store as my health food store doesn't carry it due to low demand.
The reason why they don't want you to use the concentrated form is because it has other additional ingredients that you don't need, besides being watered down. But the cleanse that I followed, the editors note suggested organic lemon juice and it works the same as squeezed lemons.