How Do Antibiotics Work?

Feb 16, 2023


Alto Pharmacy

With the discovery of antibiotics in the 1920s, once fatal illnesses including bacterial pneumonia, meningitis, and strep throat became curable. These life-saving medications remain in wide use today, treating severe infections or reducing the severity of symptoms.  

It’s very important to take antibiotics as prescribed. Like many powerful treatments, they have both benefits and risks. Improper or unnecessary use can cause avoidable side effects or make an antibiotic ineffective over the long term.

Read on to learn more about antibiotics and how to take them safely. 

What are antibiotics?

Antibiotics help your body fight a bacterial infection. Some completely kill the bacteria that caused the infection while others slow the bacteria’s growth. They are primarily derived from bacteria and fungi.

The age of antibiotics began with the Scottish physician Alexander Fleming’s accidental discovery of penicillin in 1928. Upon returning to his lab after a vacation, he observed that the emergence of mold appeared to inhibit the growth of staphylococcus, the bacteria that causes staph infections.

That mold was the origin of penicillin, the first type of antibiotic used in medicine. It became available for treatment in 1942, ushering in a new era of public health.

Antibiotics may be taken orally, as a tablet, capsule, or liquid; topically, as a cream or ointment; or intravenously. The majority are only available with prescription, but some topical antibiotics are available over the counter.

Most antibiotics are taken for 7-14 days. They begin to work immediately, but it typically takes at least several days before their benefits are experienced. The specific timing varies by antibiotic.

What do antibiotics treat?

While antibiotics are a powerful treatment for bacterial infections, they are not appropriate for all illnesses. Overuse of antibiotics has become a growing concern in recent decades. The Centers for Disease Control and Prevention (CDC) estimates that about one-third of all antibiotic use in the U.S. is unnecessary.

Antibiotics are only effective against infections caused by bacteria. This includes many ear infections, strep throat, urinary tract infections (UTIs), and some sinus infections.

They are not effective against viral infections, including:

  • The common cold and flu

  • All cases of a sore throat except strep throat

  • Most cases of bronchitis

  • Many sinus infections

The symptoms of viruses and bacterial infections frequently overlap. Receiving an accurate diagnosis is important for reducing antibiotic overuse. Your healthcare provider can identify the cause of your illness and recommend appropriate treatment.

Examples of commonly prescribed antibiotics

There are more than 100 antibiotics currently available to treat bacterial infections. Here’s an overview of the most commonly prescribed antibiotic classes, as well as potential side effects and medication interactions.


Penicillins such as penicillin V, penicillin VK, amoxicillin, dicloxacillin, and ampicillin are often used to treat strep throat, staph infections, skin conditions, ear infections, and kidney infections.

Potential side effects include nausea, diarrhea, abdominal pain, and headaches. Some people have an allergic reaction to penicillin, experiencing rashes, hives, and difficulty breathing. Seek emergency medical treatment if you experience the above signs of an allergic reaction after taking a penicillin.

Potential medication interactions include oral birth control and warfarin (Coumadin®, Jantoven®).


Cephalosporins such as cephalexin, cefaclor, and cefdinir are often used to treat skin infections, soft tissue infections, sinusitis, UTIs, and other bacterial infections.

These medications are frequently prescribed for those who are allergic to penicillins. However, other antibiotics including cephalosporins can also cause allergic reactions. Be sure to tell your doctor and pharmacist about any suspected medication allergies, even if you think it’s unrelated.


Anti-inflammatory tetracyclines such as doxycycline, minocycline, and tetracycline are often used to treat chest, urethral, and pelvic infections as well as inflammatory skin conditions, including acne and rosacea. Doxycycline is one of the most common treatments for lyme disease.

These medications, particularly doxycycline, may inflame or irritate the esophagus. Sitting or standing upright when taking doxycycline can help prevent this side effect, as well as staying hydrated. Doxycycline is also known to cause photosensitivity, making individuals more susceptible to sunburn, so it is often recommended to avoid sun exposure during treatment.

Minocycline is associated with more side effects than doxycycline, including autoimmune reactions, dizziness, and headaches.

Tetracyclines have been known to interact with diuretics, lithium, anticonvulsants, colestipol, and oral birth control.


Anti-inflammatory macrolides such as erythromycin, clarithromycin, and azithromycin are often used to treat skin conditions such as acne and rosacea; respiratory infections; and chlamydia. They are frequently prescribed for those who are allergic to penicillins or cephalosporins. 

These medications are known to interact with a variety of medications, including calcium channel blockers, anticoagulants such as warfarin, the heart medications amiodarone and digoxin, tricyclic antidepressants, anticonvulsants, benzodiazepines, lithium, and others.


Fluoroquinolones such as ciprofloxacin, moxifloxacin, and levofloxacin are often used to treat UTIs, bowel infections, and pneumonia, as well as more serious bacterial infections or infections that don’t respond effectively to other treatments. They have a high potential for antimicrobial resistance, which means they can lose their effectiveness over time as bacteria develop the ability to fight them. For this reason, it is especially important that these medications are taken only when needed.

In rare cases, fluoroquinolones may cause serious side effects, including tendon rupture and heart conditions. Older adults are most vulnerable to these effects. Always speak with your doctor about the risks and benefits of a potential medication.


Sulfonamides, or sulfa medications, such as sulfamethoxazole, sulfadiazine, and sulfasalazine are used to treat a wide range of bacterial infections. The combination of sulfamethoxazole and trimethoprim (Bactrim®) is one of the most common treatments for UTIs.

Common topical forms of sulfonamides include sulfacetamide and silver sulfadiazine.

Preventing medication interactions

Food, other medications, and supplements may affect the absorption of some antibiotics, particularly tetracyclines, macrolides, and fluoroquinolones.

To prevent interactions and side effects associated with these antibiotics, speak with your doctor or pharmacist about the best way to take your medication. Here are some helpful questions to ask:

  • Can I take this antibiotic with other medications?

  • Should I avoid certain foods or drinks when taking this antibiotic?

  • After taking this antibiotic, how long should I wait before eating?

  • Are there known potential medication interactions? If so, what signs should I watch for?

Antimicrobial resistance

Antimicrobial resistance occurs when bacteria become resistant to an antibiotic, making certain infections more difficult to treat.

This is a natural process to an extent, as bacteria constantly search for new ways to fight an antibiotic — as do fungi for antifungal medications — but inappropriate use of antibiotics is a leading cause of resistance.

The most impactful step you can take to prevent resistance is to take antibiotics only when necessary. Never use them for viral infections including the cold, flu, or a sore throat that isn’t strep.

Always take an antibiotic exactly as prescribed. This includes:

  • Taking the right dose

  • Taking all doses on time

  • Completing the course of treatment

Even if you feel better after several days, it’s important to continue taking the medication for as many days as your doctor has prescribed. Taking medication shorter or longer than directed may also contribute to resistance.

Liquid antibiotics often have specific storage instructions. Some require refrigeration while others should be stored at room temperature. Be sure to safely discard any remaining medications. Expired liquid antibiotics should never be used.

A better pharmacy makes all the difference

Our pharmacists are here to provide any information you need to take your antibiotics safely, including how to prevent medication interactions or side effects. Plus, we’ll hand-deliver your medications right to your doorstep — for free. 

To get started, reach out through in-app messaging or by phone at 1-800-874-5881.

This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or another qualified health provider with any questions you may have regarding a medical condition.

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