Citric acid is one of the acids that activate drops of MMS, thus it doesn't need to be avoided. What you should avoid is ascorbic acid (vitamin C). Jim Humble mentions Coke and other soft drinks to make it taste better.
What I do is test my desired drink with LaMotte high-range Chlorine Dioxide test strips. I just put a dose of MMS1, and whatever drink I want to test, in a small glass jar and measure the PPM with the test strips. Then I put the lid on the jar, wait an hour, and test again. If the PPM hasn't dropped, it means the CLO2 hasn't been reduced by the drink, and it should be fine to use.
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