(CNN) Doctors prescribe antibiotics to treat bacterial and fungal infections, but some people in the United States are using antibiotics without a doctor's prescription. That's a public health problem that can increase drug resistance and make it harder to treat infections, according to a study published this week in the medical journal Annals of Internal Medicine.

Researchers gathered data on nonprescription antibiotic use in the United States from 31 studies between 2000 and 2019 and focused on four main populations: patients outside a health care setting, patients in health care settings, Hispanic populations and injection drug users.

Nonprescription antibiotic use included obtaining, storing, taking or intending to take antibiotics without medical guidance. The prevalence of nonprescription antibiotic use varied from 1% (clinic patients) to 66% (Latino migrant workers), while the intention to use nonprescription antibiotics was 25% in the only study that asked. The storage of antibiotics for future use ranged from 14% to 48% across all the groups studied.

"We know that people are using antibiotics that weren't prescribed to them, which isn't safe and isn't good for their health. So in order to tackle the problem we absolutely had to know what was already out there in the literature so we could figure out what the gaps are," said study author Dr. Barbara Trautner, an infectious diseases clinician-investigator at the Baylor College of Medicine and the Houston Veterans Affairs Medical Center, who's affiliated with the Center for Innovations in Quality, Effectiveness and Safety.

One of these gaps was figuring out what factors drove people to self-treat and use nonprescription antibiotics. The studies cited several factors including poor health-care access, long wait times at the doctor's office, costs of antibiotics and doctor visits, lack of transportation and embarrassment about getting treatment for sexually transmitted infections.

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