Thank you for your inquiry.
Unfortunately, I am not sure that I can be of very much help to you. There is very little information in the medical literature on citric acid intolerance. In fact, there is no evidence whatsoever that citric acid can cause an allergy in the true sense of the word. That is, via an immunologic response in the same manner that, for example, peanuts do.
There are rare reports in medical literature of an intolerance to citric acid (1, 2), but these occur in the older literature, and there has been nothing that I could find recently to confirm this information. In the few scattered cases where citric acid seemed to produce a reaction it found that the reaction was not, as mentioned, truly allergic, but rather perhaps an irritant response.
In addition, it would be very unusual for a simple chemical like this to produce a true allergic response. Also, since citric acid is ubiquitous, found in many foods, if a person were truly allergic to citric acid, they would react to a very large diverse group of foods, not only citrus fruits but berries and a number of vegetables. Also addition, since the substance is used quite commonly as an additive and ripening agent, it would be very difficult to avoid. Since allergic reactions occur to minute amounts of the agent responsible, a true allergy to citric acid would make the person would be susceptible to having repeated, perhaps life-threatening allergic reactions because of its ubiquitous presence.
Nonetheless, it is certainly possible that intolerance to citric acid in larger amounts can occur. If so, the only treatment would be avoidance.
As to the effect of citric acid on chemotherapies, I could find nothing in the medical literature about this topic. I am not personally aware of citric acid interfering with chemotherapy in any way.
Thus, unfortunately, I can give you very little help regarding your question, and the only treatment for a true allergy to citric acid would be avoidance of all foods containing it. As you are well aware, this would be extremely difficult, necessitating, if it was a true allergy, the avoidance of all sources including berries.
In summary, although it is certainly feasible that a person could have an intolerance to higher concentrations of citric acid, I have not been able to find any case of a true allergy to this substance. This is important because when patients have true allergies, they respond to minute amounts of the substance responsible, and therefore could not tolerate any source of citric acid. On the other hand, patients who are intolerant can often tolerate smaller doses, having only a reaction to a larger amount.
Finally, I am not aware of and could not find a reference to any adverse effect of citric acid on any form of chemotherapy. However, I am not an expert in this regard, and I noticed that you are writing from Cleveland Clinic. It may be that an oncologist at your institution has more information in this regard.
Thank you again for your inquiry and we hope this response is helpful to you.
1) Allergy Volume 49, Issue 1, pages 31–37, January 1994.
2) Pediatr Allergy Immunol. 1993 Aug;4(3):123-9.
Phil Lieberman, M.D.