Those bitten by the ‘travel bug’ risk getting another type of bug—the drug-resistant kind. But trying to fight off those bacteria with drugs may make things a whole lot worse.

In a series of studies, Finnish researchers confirmed that those traveling to exotic locations—places with poor hygiene and free-flowing antibiotics—often bring home drug-resistant bacteria in their intestines (with or without symptoms). But the people who took antibiotics while exploring those locales came back with the most extensively drug-resistant cargo.

The findings, published in the journal Travel Medicine and Infectious Disease, suggest that taking antibiotics while abroad may be far more dangerous than most travelers know. After all, it’s common for world explorers to preemptively pack antibiotics, such as ciprofloxacin, for common ailments, like travelers diarrhea, the authors note.

But the findings make a lot of sense, the authors argue. “Antibiotics appear to play three vicious tricks on travelers,” they conclude:

Firstly, they disrupt intestinal colonization resistance and make space for newcomers; secondly, they only favor colonization by bacteria resistant to the antibiotic taken; and last but not least, [because] resistant bacteria [tend] to be co-resistant to other antibiotics, users end up selecting the most resistant bacteria from the surroundings.

For their work, the researchers collected poop samples from 430 Finns before and after they traveled abroad. The researchers sifted through the samples looking for extended-spectrum beta-lactamase–producing Enterobacteriaceae (ESBL-PE). Basically, these are gut bacteria that have become resistant to a wide variety of penicillins and cephalosporins, such as ceftazidime, cefotaxime, and ceftriaxone.

Twenty-one percent of travelers (90 people) came back with an ESBL-PE. The people most likely to come back with one had traveled to South and Southeast Asia, Africa, and Latin America. Having traveler’s diarrhea and taking antibiotics also upped the risk. For instance, among the people who went to South Asia, got diarrhea, and took antibiotics, 80 percent came back with an ESBL-PE.

When the researchers dug deeper into the data, the picture looked even grimmer for those who had taken antibiotics. Among those who returned with an ESBL-PE but did not take antibiotics, about 37 percent of the ESBL-PEs were also resistant to the common treatment for ESBL bacteria: a class of drugs called fluoroquinolones, which includes ciprofloxacin.

Among those who did take cipro or some other fluoroquinolone while on their trip, 95 percent of their ESBL-PE were also resistant to fluoroquinolones.

The researchers say the findings should give travelers pause before popping antibiotics while away. Moreover, the researchers also note that most cases of traveler’s diarrhea clear up on their own with a little time and can be treated with simple over-the-counter medications, such as loperamide (Imodium).

Travel Medicine and Infectious Disease, 2017. DOI: 10.1016/j.tmaid.2017.01.003 (About DOIs).