I have used both lemon and apple juice for substitutions and had good flushes. While I can still get grapefruit here, they are over a dollar apiece! If I need to do another flush this summer I'll be using something else too. I have done several flushes where I mixed 1/2 cup oil and 1/2 cup apple juice,drank that, then followed with about 1/4 cup fresh squeezed lemon juice. The lemon juice is very sour, but using it as a "chaser" cuts the oily feel in my mouth and reduces nausea. Be sure to rinse and spit with plain water after taking the acid juice, but don't swallow the water, or everything may come back up.