Antibiotics: How They Work, When to Use & Side Effects

What Are Antibiotics and How Do They Work?

Antibiotics are medications that kill bacteria (bactericidal) or stop their growth (bacteriostatic). They work by targeting structures or processes unique to bacterial cells, such as cell walls, protein synthesis, or DNA replication, without harming human cells. Antibiotics have no effect on viruses.

Antibiotics represent one of the most significant medical advances in human history. Before their discovery, simple bacterial infections could be fatal. Alexander Fleming's accidental discovery of penicillin in 1928 revolutionized medicine and has saved countless lives. Today, antibiotics remain essential tools in treating bacterial infections, from common urinary tract infections to life-threatening pneumonia and sepsis.

The fundamental principle of antibiotics is selective toxicity – they exploit differences between bacterial and human cells to kill or inhibit bacteria while minimizing harm to the patient. Bacteria are prokaryotes with distinct cellular structures that provide targets for antibiotics. For example, bacteria have cell walls made of peptidoglycan, a structure not found in human cells, which is why antibiotics like penicillin can destroy bacteria without harming us.

Understanding how antibiotics work helps explain why they must be used appropriately. Each class of antibiotics targets specific bacterial structures or processes, and different bacteria have varying vulnerabilities. This is why your doctor chooses a specific antibiotic for your particular infection – it's not a one-size-fits-all medication.

Bactericidal vs. Bacteriostatic Antibiotics

Antibiotics can be broadly categorized by how they affect bacteria. Bactericidal antibiotics directly kill bacteria by disrupting essential life processes. Examples include penicillins, cephalosporins, and fluoroquinolones. Bacteriostatic antibiotics stop bacteria from multiplying, allowing the immune system to clear the infection. Examples include macrolides, tetracyclines, and sulfonamides.

In practice, this distinction is often less clear-cut than it appears. Many bacteriostatic antibiotics become bactericidal at higher concentrations, and the clinical outcome depends more on the infection type, patient's immune status, and bacterial sensitivity than on whether the antibiotic is technically bactericidal or bacteriostatic.

Major Classes of Antibiotics

There are over 15 distinct classes of antibiotics, each with unique mechanisms of action. Understanding these classes helps explain why certain antibiotics are chosen for specific infections and why some people may be allergic to certain types but not others.

When Should You Take Antibiotics?

Antibiotics should only be taken when prescribed by a healthcare provider for confirmed or strongly suspected bacterial infections. They are effective for conditions like bacterial pneumonia, urinary tract infections, strep throat, and certain skin infections. They do NOT work for viral infections like colds, flu, COVID-19, or most sore throats and coughs.

One of the most critical aspects of antibiotic use is understanding when they are – and aren't – appropriate. The widespread misuse of antibiotics for viral infections has contributed significantly to the global crisis of antibiotic resistance. When you take antibiotics for a viral infection, you gain no benefit while potentially experiencing side effects and contributing to bacterial resistance.

Many infections that people assume require antibiotics are actually caused by viruses. The common cold is always viral. Most sore throats are viral (only about 15-30% are caused by strep bacteria). Acute bronchitis is almost always viral, even when it produces colored mucus. Sinus infections often start as viral and only sometimes develop bacterial complications requiring antibiotics.

Your doctor uses several factors to determine if you need antibiotics: your symptoms, physical examination findings, the duration and severity of illness, and sometimes laboratory tests or cultures. Rapid strep tests, for example, can quickly determine if a sore throat is bacterial. For urinary tract infections, a urine culture can confirm bacterial infection and identify which antibiotics will be effective.

Infections That Typically Require Antibiotics

• Bacterial pneumonia: A serious lung infection with symptoms including high fever, productive cough, chest pain, and difficulty breathing
• Urinary tract infections (UTIs): Bacterial infections of the bladder or kidneys causing burning urination, frequency, and pelvic pain
• Strep throat: Bacterial infection confirmed by rapid test or culture, causing severe throat pain without cough
• Bacterial skin infections: Including cellulitis, impetigo, and infected wounds with spreading redness, warmth, and pus
• Bacterial sinusitis: Sinus infection lasting over 10 days or with severe symptoms and high fever
• Sexually transmitted infections: Bacterial STIs like chlamydia, gonorrhea, and syphilis require specific antibiotics

When Antibiotics Are NOT Needed

• Common cold: Always viral – antibiotics will not help and may cause harm
• Most sore throats: Usually viral unless strep test is positive
• Influenza (flu): A viral infection requiring antiviral medications if treatment is needed
• COVID-19: A viral infection requiring specific antiviral treatments, not antibiotics
• Acute bronchitis: Almost always viral, even with colored mucus production
• Most ear infections in adults: Often resolve without antibiotics; children may need them

How Should You Take Antibiotics Correctly?

Take antibiotics exactly as prescribed: at the correct times, with or without food as directed, and for the full duration. If prescribed "three times daily," space doses every 8 hours. Never skip doses, share antibiotics, or save them for later. Proper administration ensures the antibiotic reaches effective levels in your body and reduces the risk of resistance.

The way you take antibiotics significantly affects their effectiveness. Antibiotics work best when they maintain consistent levels in your bloodstream and tissues throughout the treatment period. Taking doses at irregular intervals or stopping early can allow bacteria to survive and potentially develop resistance.

When your prescription says "take three times daily," this doesn't mean with meals – it means evenly spaced throughout your waking hours, approximately every 8 hours. For "twice daily" medications, take them 12 hours apart. Some antibiotics, like azithromycin (Z-pack), are taken once daily, while others require four doses per day. Setting phone reminders can help maintain this schedule.

Food interactions vary by antibiotic. Some must be taken on an empty stomach (1 hour before or 2 hours after eating) because food reduces their absorption. Others should be taken with food to prevent stomach upset or improve absorption. Tetracyclines should not be taken with dairy products or antacids because calcium and other minerals bind to the antibiotic and reduce its effectiveness.

Tips for Taking Antibiotics Effectively

• Read the label carefully: Note dosage, timing, and food instructions
• Set reminders: Use your phone or a pill organizer to maintain consistent timing
• Take the full course: Even if you feel better after a few days, complete all prescribed doses
• Store properly: Some require refrigeration; others must stay dry and at room temperature
• Don't skip doses: If you miss a dose, take it as soon as you remember unless it's almost time for the next dose
• Avoid certain interactions: Some antibiotics interact with alcohol, dairy, or other medications

What Are the Common Side Effects of Antibiotics?

Common antibiotic side effects include diarrhea (10-25% of patients), nausea, stomach upset, and yeast infections. Most side effects are mild and resolve after treatment ends. Serious but rare side effects include allergic reactions, tendon damage (fluoroquinolones), and C. difficile infection. Always complete your course unless you experience a severe reaction.

All medications have potential side effects, and antibiotics are no exception. The most common side effects relate to the gut because antibiotics affect not only the bacteria causing your infection but also the beneficial bacteria in your intestinal tract. This disruption of your gut microbiome can cause digestive symptoms and make you more susceptible to certain infections.

Diarrhea is the most frequent side effect, occurring in 10-25% of people taking antibiotics. This usually improves after treatment ends as your gut bacteria recover. Probiotics may help reduce antibiotic-associated diarrhea, though evidence varies by probiotic strain and antibiotic type. If diarrhea is severe, bloody, or persists after completing antibiotics, contact your healthcare provider as this could indicate C. difficile infection.

Women often experience vaginal yeast infections during or after antibiotic treatment because antibiotics kill the beneficial bacteria that normally keep yeast in check. Over-the-counter antifungal treatments are usually effective, but recurring or severe infections should be evaluated by a healthcare provider.

Common Side Effects by Antibiotic Class

What Is Antibiotic Resistance and Why Is It Dangerous?

Antibiotic resistance occurs when bacteria evolve to survive antibiotics that previously killed them. This happens primarily due to overuse and misuse of antibiotics. In 2019, antibiotic-resistant bacteria directly caused 1.27 million deaths globally. The WHO considers antimicrobial resistance one of the top 10 global public health threats facing humanity.

Antibiotic resistance is one of the most pressing public health crises of our time. When bacteria are exposed to antibiotics, most are killed, but some may have natural mutations that allow them to survive. These survivors reproduce, passing on their resistance genes. Over time, a population of bacteria can become completely resistant to antibiotics that once worked against them.

The problem is accelerating due to the overuse of antibiotics in human medicine, agriculture, and animal husbandry. Every time antibiotics are used – whether appropriately or not – they create selective pressure that favors resistant bacteria. The more antibiotics are used, the faster resistance develops. This is why using antibiotics for viral infections is particularly harmful: it provides no benefit while contributing to resistance.

Some bacteria have become resistant to multiple antibiotics, creating "superbugs" that are extremely difficult to treat. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to many common antibiotics and can cause serious skin, lung, and bloodstream infections. Carbapenem-resistant Enterobacteriaceae (CRE) are resistant to almost all antibiotics and have mortality rates up to 50% for serious infections.

How Antibiotic Resistance Develops

Bacteria develop resistance through several mechanisms. Some produce enzymes that break down antibiotics before they can work. Others modify their cell walls or membranes to prevent antibiotics from entering. Some develop "efflux pumps" that actively expel antibiotics from the cell. Bacteria can also alter the cellular targets that antibiotics attack, making the drugs ineffective.

Perhaps most concerning, bacteria can share resistance genes with each other through horizontal gene transfer. This means that when one bacterial species develops resistance, it can spread that resistance to other species – even to bacteria that were never exposed to the antibiotic. This is why resistant bacteria can spread rapidly through communities and healthcare facilities.

How You Can Help Combat Antibiotic Resistance

• Only take antibiotics when prescribed: Don't pressure your doctor for antibiotics for viral infections
• Complete your full course: Stopping early allows surviving bacteria to develop resistance
• Never share antibiotics: Antibiotics are prescribed for specific infections and individuals
• Don't save leftover antibiotics: If you have leftover pills, you likely didn't take the full course
• Practice good hygiene: Hand washing and food safety reduce infection spread
• Stay up to date on vaccines: Preventing infections reduces the need for antibiotics

Can You Drink Alcohol While Taking Antibiotics?

With most antibiotics, moderate alcohol consumption won't cause serious problems but may increase side effects like nausea and drowsiness. However, with metronidazole and tinidazole, alcohol must be strictly avoided as it causes severe nausea, vomiting, and rapid heartbeat. Always check your specific medication's instructions and ask your pharmacist.

The interaction between alcohol and antibiotics is often misunderstood. While the blanket advice to avoid alcohol with all antibiotics is overly cautious for many medications, there are legitimate reasons for concern with specific drugs, and alcohol can interfere with your recovery in other ways.

The most serious alcohol-antibiotic interaction occurs with metronidazole (Flagyl) and tinidazole (Tindamax). These medications block an enzyme that normally breaks down alcohol, causing acetaldehyde to accumulate in your body. This results in a severe reaction including nausea, vomiting, flushing, rapid heartbeat, and headache – similar to the reaction caused by the drug Antabuse used to treat alcoholism. You should avoid alcohol during treatment and for at least 48-72 hours after completing these medications.

Other antibiotics with notable alcohol interactions include certain cephalosporins, which can cause similar but milder reactions, and linezolid, which can cause dangerous blood pressure spikes when combined with certain alcoholic beverages (particularly tap beer and red wine) due to tyramine content.

General Considerations About Alcohol During Antibiotic Treatment

Even when there's no direct drug interaction, drinking alcohol while taking antibiotics may not be wise. Alcohol can impair your immune system's ability to fight infection. It can worsen common antibiotic side effects like nausea, dizziness, and stomach upset. Alcohol can also disrupt sleep, which is important for recovery. If you're sick enough to need antibiotics, it's generally best to let your body focus on healing.

Do Antibiotics Affect Birth Control?

Most antibiotics do NOT significantly reduce the effectiveness of hormonal birth control. The exception is rifampin (and related drugs rifabutin and rifapentine), which can decrease birth control effectiveness. If you're taking these medications, use backup contraception. Vomiting or diarrhea from any antibiotic can also affect pill absorption.

This is one of the most common questions about antibiotics, and the answer may surprise you. For decades, women were told that antibiotics could make their birth control pills less effective. However, extensive research has shown that this concern is largely unfounded for most antibiotics.

The one clear exception is rifampin (and related drugs rifabutin and rifapentine), used primarily to treat tuberculosis and some other serious infections. Rifampin significantly increases the liver's metabolism of hormonal contraceptives, reducing their blood levels and effectiveness. If you're taking rifampin, you should use additional contraception (like condoms) during treatment and for at least one month after completing the medication.

For all other commonly prescribed antibiotics – including amoxicillin, azithromycin, doxycycline, ciprofloxacin, and others – high-quality studies have not found significant effects on hormonal contraceptive effectiveness. The American College of Obstetricians and Gynecologists states that additional contraception is not needed with most antibiotics.

When Backup Contraception May Be Needed

• Rifampin and related drugs: Use backup contraception during treatment and for 1 month after
• Severe vomiting: If you vomit within 2 hours of taking a birth control pill, absorption may be incomplete
• Severe diarrhea: May reduce absorption of hormonal contraceptives
• Extended antibiotic courses: Discuss with your healthcare provider if you have concerns

Why Is It Important to Complete the Full Course?

Completing your antibiotic course ensures all bacteria are killed. Stopping early when you feel better may leave some bacteria alive – often the most resistant ones. These survivors can multiply and cause a relapse that's harder to treat. However, newer research suggests shorter courses may work for some infections – follow your doctor's specific instructions.

The traditional advice to "always finish your antibiotics" is being re-examined by medical researchers, but the core principle remains important. When you take antibiotics, they kill the most susceptible bacteria first. If you stop treatment early, you may feel better because the bacterial load has decreased, but some bacteria may survive – and these survivors are often the most resistant.

These surviving bacteria can then multiply, potentially causing a relapse. Worse, because they survived antibiotic exposure, they may be harder to treat. You might need a different antibiotic, a higher dose, or a longer course. In some cases, the infection could become completely resistant to the original antibiotic.

However, medical thinking on antibiotic course length is evolving. Research has shown that for some infections, shorter courses are just as effective as longer ones. For uncomplicated urinary tract infections, 3-5 days may be sufficient instead of the traditional 7-10 days. For some cases of pneumonia, 5 days works as well as 10 days. These shorter courses reduce side effects, cost, and potentially resistance development.

The Bottom Line on Course Length

Follow your prescriber's specific instructions. If they prescribe 7 days, take all 7 days. If they prescribe 5 days, don't extend it to 7. Medical recommendations are based on research for specific infections. Your doctor has considered factors like infection type, severity, your overall health, and local resistance patterns when determining the appropriate duration.

Are Antibiotics Safe for Children?

Antibiotics are safe and often necessary for children with bacterial infections, but they're overprescribed for viral illnesses in pediatric populations. Children are more likely to experience diarrhea from antibiotics. Some antibiotics (tetracyclines, fluoroquinolones) have age restrictions due to effects on developing bones and teeth. Dosing is based on weight, not age.

Children get sick frequently, and parents often feel pressure to get antibiotics. However, most childhood illnesses – including colds, most ear infections, sore throats (except strep), and bronchitis – are viral and don't require antibiotics. Unnecessary antibiotic use in children contributes to resistance and can cause side effects.

When children do need antibiotics for bacterial infections, the medications are generally safe when used appropriately. However, some antibiotics are not suitable for children. Tetracyclines can cause permanent tooth discoloration and shouldn't be used in children under 8 years old. Fluoroquinolones can potentially affect growing cartilage and bones, so they're typically reserved for serious infections where other options aren't suitable.

Antibiotic dosing in children is based on weight, not age, to ensure adequate medication levels without overdosing. This is why pediatric antibiotics often come as liquids with dosing syringes. It's crucial to use the correct measuring device – kitchen spoons are not accurate – and complete the full course as prescribed.

Tips for Giving Antibiotics to Children

• Use the correct measuring device: Always use the provided dropper, syringe, or measuring cup
• Maintain the schedule: Set reminders, especially for school-age children
• Check storage requirements: Some liquid antibiotics require refrigeration
• Complete the course: Even if your child feels better and wants to stop
• Watch for reactions: Report rashes, severe diarrhea, or difficulty breathing immediately
• Consider probiotics: May help reduce antibiotic-associated diarrhea in children