DES MOINES, Iowa (WHO) — It’s something many people do when feeling sick, take an old antibiotic before seeing a doctor. The consensus across the medical field, however, is that’s a bad idea.

MercyOne outpatient pharmacy Services Manager Michelle Welsh said it’s a really bad idea on several fronts to take antibiotics for any purpose other than their intended use.

In fact, she said it’s a bad idea to have leftover antibiotics to begin with because the patient didn’t finish the entire course, which means their body likely didn’t kill the entire previous infection.

She added it’s also not smart to take antibiotics without a prescription because different drugs treat different bacteria. They’re not one-size-fits-all. Some antibiotics work for some things while others don’t.

“You should always consult with a doctor to see if they’re needed,” Welsh said. “And if they are, they will determine which is the right antibiotic for that type of infection because you may have an ear infection but it could be caused by different types of bacteria, and the doctor will be able to assess that and determine which antibiotic is going to kill off that particular type of bacteria.”

Antibiotics also don’t work against viruses. That includes the flu, COVID-19 and RSV.

Welsh said if people take antibiotics when they are not supposed to it, can lead to resistance where certain infections get strong enough that the drugs don’t work.