Antibiotics: Types, Uses & How to Take Them Safely

What Are Antibiotics and How Do They Work?

Antibiotics are medications that kill bacteria or stop them from multiplying. They work through various mechanisms including destroying bacterial cell walls, blocking protein production, and interfering with DNA replication. Antibiotics have no effect on viruses, which is why they don't help with colds or flu.

The term "antibiotics" is often used interchangeably with "penicillin" in everyday language, but penicillin is actually just one type of antibiotic among many. Antibiotics represent one of the most important medical discoveries in history, having saved countless millions of lives since Alexander Fleming discovered penicillin in 1928. Before antibiotics, simple bacterial infections could be fatal, and surgical procedures carried enormous risks of deadly infections.

The way antibiotics work depends on their class and mechanism of action. Some antibiotics are bactericidal, meaning they actively kill bacteria. Others are bacteriostatic, meaning they stop bacteria from reproducing, giving the immune system time to eliminate the infection. The choice between these types depends on the severity of the infection, the patient's immune status, and the specific bacteria involved.

Different antibiotics target different parts of the bacterial cell. Beta-lactam antibiotics (including penicillins and cephalosporins) attack the bacterial cell wall, causing the bacteria to burst. Macrolides and tetracyclines interfere with protein synthesis, preventing bacteria from making the proteins they need to survive and reproduce. Fluoroquinolones target bacterial DNA replication machinery, stopping bacteria from copying their genetic material.

Narrow-Spectrum vs. Broad-Spectrum Antibiotics

Antibiotics can be classified by the range of bacteria they affect. Narrow-spectrum antibiotics target only a few specific types of bacteria, while broad-spectrum antibiotics are effective against many different bacterial species. This distinction has important clinical implications that affect treatment decisions.

Narrow-spectrum antibiotics are generally preferred when the specific bacteria causing the infection is known. They cause less disruption to the body's normal bacterial flora, which includes beneficial bacteria living in the gut, skin, and other areas. When these helpful bacteria are preserved, patients experience fewer side effects like diarrhea and yeast infections. Examples of narrow-spectrum antibiotics include penicillin V and certain older antibiotics that target gram-positive bacteria specifically.

Broad-spectrum antibiotics become necessary when the infection is severe, when the specific bacteria hasn't been identified, or when multiple types of bacteria may be involved. However, their wider range of action also means they kill more of the body's beneficial bacteria, leading to more side effects and potentially creating conditions for opportunistic infections like Clostridioides difficile colitis.

When Do You Actually Need Antibiotics?

Antibiotics are needed for bacterial infections such as strep throat, urinary tract infections, bacterial pneumonia, skin infections, and certain sexually transmitted infections. They are NOT needed for viral infections like colds, flu, most sore throats, bronchitis, or COVID-19. Your doctor must determine if your infection is bacterial before prescribing antibiotics.

One of the most common misconceptions about antibiotics is that they help with any infection. In reality, antibiotics are completely ineffective against viral infections, which include the common cold, influenza (flu), most cases of bronchitis, many sinus infections, and most sore throats. Taking antibiotics for these conditions provides no benefit while exposing you to potential side effects and contributing to antibiotic resistance.

The body's immune system is remarkably capable of fighting off most viral infections without medication. When you have a cold or flu, your symptoms are actually largely caused by your immune system's response to the virus, not the virus itself. This is why these illnesses typically resolve within 7-10 days regardless of treatment. Supportive care like rest, fluids, and over-the-counter symptom relievers are the appropriate approach for most viral illnesses.

However, there are many situations where antibiotics are genuinely necessary and potentially lifesaving. Bacterial infections that typically require antibiotic treatment include strep throat (caused by Streptococcus bacteria), urinary tract infections, bacterial pneumonia, certain skin infections like cellulitis, bacterial meningitis, sepsis (bloodstream infections), and many sexually transmitted infections like chlamydia and gonorrhea.

Determining whether an infection is bacterial or viral isn't always straightforward. Some infections can be caused by either bacteria or viruses, and symptoms may overlap significantly. This is why laboratory tests are sometimes necessary before starting antibiotic treatment. Rapid strep tests, urine cultures, blood cultures, and other diagnostic tools help doctors make accurate diagnoses and choose appropriate treatments.

What Are the Different Types of Antibiotics?

The main classes of antibiotics include penicillins (the most commonly used), cephalosporins, tetracyclines, macrolides, fluoroquinolones, and sulfonamides. Each class works differently and targets different types of bacteria. Your doctor chooses the antibiotic based on the type of infection, the bacteria involved, and your individual health factors.

Understanding the different classes of antibiotics helps explain why certain antibiotics are prescribed for specific infections and why some people may need alternatives due to allergies or other factors. Each antibiotic class has unique properties, mechanisms of action, and spectrum of activity that determine its clinical applications.

Penicillins

Penicillins are the most widely used class of antibiotics and include several important medications. The original penicillin V (phenoxymethylpenicillin) remains a first-line treatment for many common infections including strep throat, certain skin infections, and some respiratory infections. Penicillin V has a narrow spectrum, meaning it targets a limited range of bacteria, which actually makes it preferable when the specific bacteria is known.

Amoxicillin is a broader-spectrum penicillin that works against a wider range of bacteria. It's commonly prescribed for ear infections, respiratory tract infections, and dental infections. When combined with clavulanic acid (as in Augmentin), amoxicillin becomes effective against bacteria that would otherwise be resistant because they produce enzymes that destroy penicillin.

Other important penicillins include flucloxacillin, which is particularly effective against skin infections caused by staphylococcal bacteria, and pivmecillinam, which is specifically used for uncomplicated urinary tract infections.

Cephalosporins

Cephalosporins are closely related to penicillins and work through similar mechanisms. They are classified into generations (first through fifth), with later generations generally having broader spectrum activity. Because of their relationship to penicillins, people with severe penicillin allergies may also react to cephalosporins, though cross-reactivity is less common than previously thought.

Cephalosporins are commonly used for urinary tract infections, skin infections, and respiratory infections. They may be prescribed when penicillins have failed or when broader coverage is needed. Examples include cefadroxil (oral) and ceftriaxone (injectable).

Tetracyclines

Tetracyclines are broad-spectrum antibiotics effective against many different bacteria. Common examples include doxycycline, tetracycline, and lymecycline. These antibiotics are frequently used for respiratory infections, particularly those caused by atypical bacteria like Mycoplasma, as well as for acne, Lyme disease, and certain sexually transmitted infections.

An important consideration with tetracyclines is that they should not be given to children under eight years of age or to pregnant women, as they can affect developing bones and teeth, potentially causing permanent tooth discoloration. Tetracyclines also increase sensitivity to sunlight, so sun protection is essential during treatment.

Macrolides

Macrolides include azithromycin, erythromycin, clarithromycin, and roxithromycin. They work by inhibiting bacterial protein synthesis and are effective against many of the same bacteria as penicillins, making them valuable alternatives for patients with penicillin allergies.

Azithromycin is particularly notable for its convenient dosing—often just three to five days for many infections compared to longer courses with other antibiotics. Macrolides are commonly used for respiratory infections, skin infections, and certain sexually transmitted infections. They are also effective against atypical bacteria that cause conditions like "walking pneumonia."

Fluoroquinolones

Fluoroquinolones (also called quinolones) include ciprofloxacin, levofloxacin, and moxifloxacin. These are powerful broad-spectrum antibiotics that work by interfering with bacterial DNA replication. They are reserved for serious infections including complicated urinary tract infections, certain types of pneumonia, and some gastrointestinal infections.

Due to concerns about serious side effects including tendon damage, nerve damage, and mental health effects, fluoroquinolones are now typically reserved for situations where other antibiotics are not appropriate. They are generally not given to children and adolescents because of potential effects on developing cartilage, except in specific circumstances.

Trimethoprim and Sulfonamides

Trimethoprim is commonly used alone or in combination with sulfamethoxazole (a sulfonamide) for urinary tract infections and certain respiratory infections. The combination, often called co-trimoxazole, provides broad coverage and is particularly useful for specific types of pneumonia in immunocompromised patients.

How Should You Take Antibiotics Correctly?

Take antibiotics exactly as prescribed: the right dose, at the right times, for the full duration. Some should be taken with food, others on an empty stomach. Complete the entire course even if you feel better. Skipping doses or stopping early can lead to treatment failure and antibiotic resistance.

Proper antibiotic use is crucial for treatment success and for preventing the development of resistant bacteria. When you receive an antibiotic prescription, understanding how to take it correctly can make the difference between a successful cure and treatment failure.

The first and most important rule is to take your antibiotic exactly as prescribed by your doctor. This includes the correct dose, the correct frequency (how many times per day), and for the full duration specified. When doctors prescribe antibiotics, they consider the type of infection, the bacteria likely involved, and the time needed to completely eliminate the infection.

Timing matters with antibiotics. If your prescription says to take the medication three times daily, try to space doses approximately eight hours apart to maintain consistent levels of the antibiotic in your bloodstream. If it's twice daily, aim for doses about 12 hours apart. Consistent timing helps ensure the antibiotic concentration stays high enough to effectively fight the infection.

Food interactions vary between different antibiotics. Some antibiotics work better when taken on an empty stomach (one hour before or two hours after eating), while others should be taken with food to reduce stomach upset or improve absorption. Common antibiotics that should be taken with food include amoxicillin-clavulanate and metronidazole. Always check your prescription label or ask your pharmacist about food interactions.

Why Completing the Full Course Matters

Even if you feel completely better after a few days of antibiotics, it's essential to complete the entire prescribed course. Here's why: when you start antibiotics, they begin killing bacteria, and you may feel improvement quickly as bacterial numbers decrease. However, not all bacteria are killed at the same rate. Some bacteria take longer to be eliminated, and stopping early leaves these survivors behind.

These surviving bacteria can multiply and cause the infection to return, often more severely. Even more concerning, the bacteria that survive may be those with some degree of natural resistance to the antibiotic. By stopping treatment early, you're essentially selecting for more resistant bacteria, which can then spread to others and contribute to the broader problem of antibiotic resistance.

There are rare exceptions where doctors may advise shorter courses than traditionally used, based on newer research. However, any decision to shorten treatment should come from your healthcare provider, not from you deciding to stop because you feel better.

What Are Common Side Effects of Antibiotics?

Common antibiotic side effects include gastrointestinal problems (diarrhea, nausea, stomach pain), yeast infections (thrush), and skin rashes. These occur because antibiotics also kill beneficial bacteria in your body. Serious but rare side effects include severe allergic reactions (anaphylaxis), Clostridioides difficile infection, and tendon problems with fluoroquinolones.

Understanding potential side effects helps you recognize normal reactions versus problems that require medical attention. While most antibiotic side effects are mild and resolve after treatment ends, some can be serious and require prompt medical care.

The most common side effects relate to the gastrointestinal system. Antibiotics don't distinguish between harmful bacteria causing your infection and the beneficial bacteria living in your gut. When these helpful gut bacteria are disrupted, it commonly causes loose stools or diarrhea, nausea, stomach cramps, or bloating. These symptoms are usually mild and resolve after completing treatment.

Yeast infections are another common consequence of antibiotic use, particularly in women. The vagina normally contains a balance of bacteria and yeast. When antibiotics reduce the bacterial population, yeast (especially Candida) can overgrow, causing vaginal itching, discharge, and discomfort. Oral thrush (yeast infection in the mouth) can also occur. These infections are treatable with antifungal medications.

Skin reactions ranging from mild rashes to more serious reactions can occur with any antibiotic. Simple rashes may resolve on their own or with antihistamines. However, if you develop hives (raised, itchy welts), facial or throat swelling, difficulty breathing, or fever along with a skin rash, stop the antibiotic and seek immediate medical attention as these could indicate a serious allergic reaction.

Serious Side Effects Requiring Medical Attention

Clostridioides difficile infection (C. diff): This serious intestinal infection can occur when antibiotics severely disrupt normal gut bacteria, allowing C. diff bacteria to overgrow. Symptoms include severe, watery diarrhea (sometimes bloody), fever, abdominal pain, and loss of appetite. C. diff can be life-threatening and requires specific antibiotic treatment. Contact your doctor immediately if you develop severe or bloody diarrhea during or after antibiotic treatment.

Anaphylaxis: This severe, potentially life-threatening allergic reaction can occur with any antibiotic but is most common with penicillins and sulfonamides. Symptoms develop rapidly and may include difficulty breathing, throat tightening, widespread hives, rapid heartbeat, dizziness, and loss of consciousness. Anaphylaxis requires immediate emergency treatment with epinephrine and emergency medical care.

Tendon problems with fluoroquinolones: Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin, moxifloxacin) can cause tendon inflammation and rupture, most commonly affecting the Achilles tendon. Risk is higher in people over 60, those taking corticosteroids, and kidney, heart, or lung transplant recipients. If you experience tendon pain, swelling, or weakness while taking these antibiotics, stop the medication and contact your doctor immediately.

What Is Antibiotic Resistance and Why Should You Care?

Antibiotic resistance occurs when bacteria evolve to survive antibiotic treatment. This happens through overuse and misuse of antibiotics in humans and agriculture. Resistant bacteria cause infections that are difficult or impossible to treat, leading to longer illnesses, more deaths, and higher healthcare costs. The WHO considers antibiotic resistance one of the top 10 global health threats.

Antibiotic resistance is one of the most serious public health threats facing humanity. The World Health Organization has declared it one of the top 10 global public health threats. Without effective antibiotics, many medical advances we take for granted—including surgery, cancer chemotherapy, and organ transplants—become much more dangerous due to the risk of untreatable infections.

Resistance develops through natural evolution accelerated by human activity. Bacteria reproduce rapidly, with some species dividing every 20 minutes. During reproduction, random genetic mutations occur. Occasionally, a mutation allows a bacterium to survive an antibiotic. In the presence of antibiotics, these resistant bacteria survive while susceptible bacteria die. The resistant bacteria then multiply, and the gene for resistance can even be shared with other bacteria.

Every time antibiotics are used, there's selective pressure favoring resistant bacteria. This is why antibiotic overuse and misuse are so problematic. Taking antibiotics for viral infections (which they can't treat) still exposes your body's bacteria to the drug, selecting for resistance. Stopping antibiotics early can leave behind partially resistant bacteria. Using the wrong antibiotic or wrong dose can have similar effects.

Agricultural use of antibiotics is another major contributor to resistance. Antibiotics have been used extensively in livestock farming, both to treat and prevent infections and historically to promote growth. Resistant bacteria from animals can spread to humans through food, direct contact, or environmental contamination. Many countries have now restricted agricultural antibiotic use, but the practice continues in many parts of the world.

The Human Cost of Antibiotic Resistance

The consequences of antibiotic resistance are already being felt worldwide. Currently, antibiotic-resistant infections cause approximately 1.27 million deaths annually, according to a 2022 study published in The Lancet. By 2050, without significant intervention, this number could rise to 10 million deaths per year—more than cancer kills today.

Resistant infections mean longer illnesses, more hospitalizations, the need for more expensive and toxic alternative treatments, and higher mortality rates. Infections that were once easily treatable can become life-threatening. Procedures that carry infection risk become more dangerous, potentially limiting surgical options for many patients.

Some bacteria have become resistant to almost all available antibiotics, creating what the media often calls "superbugs." Examples include methicillin-resistant Staphylococcus aureus (MRSA), extensively drug-resistant tuberculosis (XDR-TB), and carbapenem-resistant Enterobacteriaceae (CRE). Infections with these organisms have limited treatment options and high mortality rates.

How You Can Help Combat Antibiotic Resistance

• Only take antibiotics when prescribed by a doctor for a confirmed bacterial infection
• Never demand antibiotics if your doctor says they aren't needed
• Complete the full prescribed course even if you feel better
• Never share antibiotics with others or use leftover antibiotics
• Practice good hygiene including hand washing, to prevent infections
• Keep vaccinations up to date to prevent infections that might otherwise need antibiotics
• Practice safe food handling to prevent foodborne bacterial infections

Can You Take Antibiotics During Pregnancy or While Breastfeeding?

Some antibiotics are safe during pregnancy and breastfeeding, while others should be avoided. Penicillins, cephalosporins, and certain macrolides are generally considered safe. Tetracyclines and fluoroquinolones should be avoided during pregnancy. Always tell your doctor if you are or might be pregnant, or if you are breastfeeding, before taking any antibiotic.

Pregnancy and breastfeeding require special consideration when it comes to antibiotic use. The developing fetus and nursing infant can be affected by medications taken by the mother, so the risks and benefits must be carefully weighed. However, untreated bacterial infections during pregnancy can also harm both mother and baby, so antibiotics are sometimes necessary.

Penicillins and cephalosporins are generally considered safe during pregnancy and are often the first choice when antibiotics are needed. These antibiotics have been used for decades in pregnant women with no evidence of harm to the fetus. Certain macrolides, particularly azithromycin, are also considered relatively safe options.

Tetracyclines should be avoided during the second and third trimesters of pregnancy (and in children under 8) because they can affect fetal bone development and cause permanent tooth discoloration. Fluoroquinolones are also generally avoided due to potential effects on fetal cartilage development, though they may be used when no safer alternative exists for a serious infection.

For breastfeeding mothers, most antibiotics pass into breast milk in small amounts. Penicillins, cephalosporins, and macrolides are generally considered compatible with breastfeeding. The infant may experience mild GI effects like diarrhea. More importantly, if you need antibiotics, treatment of your infection is essential for your health and your ability to care for your baby. Discuss options with your healthcare provider.

Why Am I Not Getting Better with Antibiotics?

Antibiotics may not work because the infection is viral (antibiotics don't work on viruses), the wrong antibiotic was prescribed for the bacteria involved, the dose was too low, the treatment course was too short, or the bacteria are resistant. If you're not improving after 48-72 hours of antibiotics, contact your doctor.

When antibiotic treatment doesn't seem to be working, several factors could be responsible. Understanding these reasons can help you work with your healthcare provider to find an effective solution.

The most common reason antibiotics don't work is that the infection is actually caused by a virus, not bacteria. Viral infections including colds, flu, most sore throats, and many respiratory infections will not respond to antibiotics at all. If antibiotics were prescribed based on an assumed bacterial cause but the actual infection is viral, you won't see improvement from the antibiotic.

Even with bacterial infections, the prescribed antibiotic might not be effective against the specific bacteria causing your infection. This can happen when doctors must prescribe empirically (before knowing exactly which bacteria is involved) and their initial choice doesn't cover the actual pathogen. This is why doctors sometimes request cultures before starting treatment, or change antibiotics if the first choice isn't working.

Dosing and duration issues can also lead to treatment failure. If the antibiotic dose is too low, it may not achieve adequate concentrations at the infection site. Similarly, if treatment is stopped too early, the infection may not be completely cleared and can return. Resistant bacteria are more likely to survive inadequate treatment.

Antibiotic resistance itself is an increasingly common cause of treatment failure. If the bacteria causing your infection have developed resistance to the prescribed antibiotic, it won't be effective regardless of proper dosing and duration. This may require switching to a different antibiotic class.

Finally, some infections require more than antibiotics alone. Abscesses may need surgical drainage in addition to antibiotics. Some infections require longer treatment courses than initially prescribed. Certain medical conditions can impair immune function and make infections harder to clear.