Antibiotics are some of the most powerful tools in medicine. They’re prescribed to treat a variety of infections caused by bacteria, such as urinary tract infections, most ear infections, strep throat and bacterial pneumonia.

But just because they’re so effective doesn’t mean they’re meant to treat every cough, ache and sniffle. So when do you actually need to take an antibiotic? And do you really have to finish the entire course? Here’s what you need to know about antibiotics, according to experts.

First, how exactly do antibiotics work?

Antibiotics work by either killing bacteria directly or preventing their growth and spread. “Because if it can’t grow, it will die,” Hai Tran, clinical pharmacist and associate director of pharmacy at Cedars-Sinai, tells Yahoo Life.

But not all antibiotics work — i.e. fight bacteria — the same way. “Different antibiotics can accomplish this through different mechanisms, and some antibiotics are active against certain bacteria while others might work against different types of bacteria,” Jessica Smith, a clinical pharmacist who practices in infectious diseases at the Ohio State University Wexner Medical Center, tells Yahoo Life. And of course, your immune system also plays a role. “Antibiotics, along with the body’s immune system, help to eliminate infections caused by bacteria,” she says.

When shouldn't you take an antibiotic?

Again, antibiotics only work against bacterial infections. “It does not work against viruses or fungus,” says Tran. “The confusion always comes up around wintertime because of the flu. The flu is caused by a virus.”

Smith agrees, explaining that, since antibiotics are tailored to work against bacteria, they are only helpful for infections caused by bacteria. “Many infections — e.g., the common cold, COVID-19, influenza/’the flu’ — are caused by viruses, so taking an antibiotic will not help,” she says. “Antivirals — which work against viruses — can be used in certain situations to treat these viruses, but antibiotics — which work against bacteria — are not helpful.”

She adds that some infections, such as sinusitis, bronchitis or pneumonia, can be caused by either viruses or bacteria. So, Smith says, it’s important to talk to a health care provider about whether antibiotics are needed in these situations.

That said, in some cases a viral infection can turn into a bacterial infection. When you have a cold or flu for a long period of time, that can start to impact the “good bacteria” in your body and turn into a “super infection,” explains Tran.

“At that point, an antibiotic might be warranted,” she says. “Most coughs and colds are caused by viruses so you don't want to treat the patient with an antibiotic at that point. But if it persists for a week or longer and the patient continues to have fever that’s when you start worrying about a bacterial infection. So it depends on the timing. That’s why physicians ask about how long you’ve had symptoms.”

Why is it important not to use antibiotics when you don’t need them?

While it may be tempting to take antibiotics even if you don’t necessarily need them, experts say that’s a bad idea. “There are several reasons why it is important not to overuse antibiotics,” says Smith. “First, the overuse of antibiotics can promote antibiotic resistance. When bacteria are exposed to an antibiotic, they are more likely to be resistant to that same antibiotic — and sometimes, other antibiotics — in the future.”

In some cases, repeated exposure to different antibiotics can result in “super bugs,” which do not have effective treatment options, explains Smith.

Second, antibiotics often kill “good” bacteria living in our bodies in addition to the “bad” bacteria causing infection. “The good bacteria that live in our gut are important for digestion and help to protect us against infections caused by bad bacteria,” explains Smith. “When they are killed by antibiotics, bad bacteria can take over and cause problems like Clostridioides difficile (C. diff) infection. Some doctors recommend taking a probiotic with an antibiotic, which can help with certain side effects, such as diarrhea.

Finally, antibiotics have side effects, some of which can be severe, says Smith. These include nausea, diarrhea, rash, dizziness or yeast infections, according to the Centers for Disease Control and Prevention. With antibiotics in the fluoroquinolone family, which includes Cipro, rare but serious side effects can include joint swelling, tendonitis or tendon rupture.

For these reasons, says Smith, it’s important to “only use antibiotics when a bacterial infection is present and for the shortest effective duration.”

Do you always need to complete the full course of antibiotics?

“Absolutely,” says Tran. Smith agrees, saying that it’s important to use antibiotics as directed and not go rogue. “Even if you start to feel better, you should finish the antibiotic course prescribed by a health care provider to make sure the infection is fully treated and decrease the likelihood that it will return,” she says. “Stopping an antibiotic early or switching to a different antibiotic should only be done at the direction of a health care provider.”

Not finishing the entire course of antibiotics is more serious than you might think. Tran explains why: “Let’s say you stopped too soon because you felt better — you felt better because the antibiotic killed most of the bacteria at that time. But if you stop at that time, some bacteria have not completely been killed and they’ll start to grow again.”

But that doesn’t just mean the infection could come back. It can also lead to antibiotic resistance. “What could happen is that these bacteria will start figuring out ways to fight that particular antibiotic,” points out Tran. “It can lead to resistance. And now we find out the antibiotic no longer works.”

Why is it a bad idea to use a family member or friend’s antibiotic?

Antibiotics aren’t a one-size-fits-all type of medication and should only be taken as prescribed by a health care provider. “Antibiotics are chosen very carefully based on the patient being treated — e.g., age, weight, kidney function — the type of infection being treated and the bacteria causing the infection,” explains Smith.

Health care prescribers and pharmacists also check to make sure the antibiotic is prescribed at the correct dose and duration, she says, and “they look for potential issues like drug interactions and medication allergies.”

If you are sick and think you need an antibiotic, Smith says it’s always best to get evaluated by a health care provider who can determine whether an antibiotic is warranted, and if so, which one is most likely to be both safe and effective.

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