In the comments section of the BJJ Caveman keto rash page a reader named John wrote:
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I had no reoccurrence of the rash after taking Doxycycline for 11 days without any change in my diet 9 months ago (see above and be aware of rare but possible severe side effects affecting mood). In order to avoid antibiotic-associated diarrhea I took Saccharomyces boulardii together with the antibiotic.
I now do intermittent fasting daily throughout the work-week without any problems. My longest fast was 36 hours.
Maybe Doxycycline wipes out certain types of gut bacteria that can proliferate extremely well under the conditions of ketosis, but not otherwise and produce a substance that causes the rash.