I am one of those, and over the years I experimented with going on and off Nystatin a sufficient number of times to convince me that this was a real effect, not an accident or a placebo effect. Yeast overgrowths are promoted by several factors, one of which is antibiotic therapy.
Antibiotics kill off beneficial bacteria as well as pathogens.
Ordinarily, bacteria and yeasts live together in competition and the yeasts are excluded or kept to an insignificant level. When the bacteria are killed, this competition is destroyed, yeasts can invade or expand, and they often do. Once established, an intestinal yeast overgrowth might perpetuate itself, and the patient might suffer in any of very many ways from the consequences of toxic metabolites being carried in the blood stream all over the body. In principal, the prostate could be affected in this way, and prostatitis could result. As far as I know, every effort to substantiate Truss' hypothesis in controlled scientific experiments has failed.