To the Editor: We compliment Sack and his colleagues on a well designed investigation of the efficacy of prophylactic doxycycline in the prevention of travelers' diarrhea, as reported in the April 6 issue of the Journal, but we are concerned about generalizing from 18 patients treated with doxycycline. The basis for our concern is that such a small series does not exclude the potential hazards of giving the drug to large numbers of travelers who may wish to take prophylactic antibiotics.Although doxycycline may be unique since it is excreted into the small bowel, the effect of another prophylactic . . .