Antibiotics: Most Common Types and Their Side Effects Explained

Antibiotics save lives. They turn deadly infections like pneumonia or sepsis into treatable conditions. But they’re not harmless. Every time you take one, your body reacts - sometimes in ways you didn’t expect. Around 1 in 5 people who take antibiotics get stomach issues. About 10% think they’re allergic to penicillin, but most of them aren’t. And every year, over a million people die because antibiotics stop working - not because the drugs are weak, but because we’ve overused them.

How Antibiotics Actually Work

Antibiotics don’t treat viruses. They only work on bacteria. That’s why they won’t help with colds or the flu. They either kill bacteria outright (bactericidal) or stop them from multiplying (bacteriostatic). The first one, penicillin, was found by accident in 1928 when Alexander Fleming noticed mold killing bacteria in a petri dish. Today, we have dozens of types, grouped by how they attack bacteria.

Some break down bacterial cell walls. Others block protein building or DNA copying. The way they work determines which infections they’re good for - and what side effects they cause.

The 7 Most Common Antibiotic Classes

Doctors pick antibiotics based on the infection, your history, and local resistance patterns. Here are the seven classes you’re most likely to be prescribed.

Penicillins: The Original

Penicillins are the oldest and still the most common. Amoxicillin alone makes up nearly one in five antibiotic prescriptions in the U.S. It’s used for ear infections, sinusitis, strep throat, and pneumonia. Other names include ampicillin, penicillin V, and dicloxacillin.

They work by breaking down the bacterial cell wall - like popping a balloon from the inside. Side effects? Nausea (15-20%), diarrhea (5-10%), and abdominal pain. About 10% of Americans say they’re allergic to penicillin. But a 2023 JAMA study found that 9 out of 10 of those people can take it safely after proper testing. Many think they’re allergic because they had a rash as a kid - but that’s often not a true allergy.

Cephalosporins: The Penicillin Alternative

If you’re told you can’t take penicillin, you’re often given a cephalosporin. Cephalexin is the most common - it’s used for skin infections, urinary tract infections, and even some bone infections. Others include ceftriaxone (given in hospitals) and cefuroxime.

They work similarly to penicillins but are less likely to cause cross-reactions. Only 1-3% of people allergic to penicillin react to cephalosporins. Side effects are mild: diarrhea, nausea, and occasional rash. Severe reactions like Stevens-Johnson syndrome are extremely rare - fewer than 1 in 10,000.

Tetracyclines: For Acne and Tick Bites

Doxycycline is the star here. It’s prescribed for acne, Lyme disease, and some types of pneumonia. It’s also used to prevent malaria in travelers. It works by blocking bacteria’s protein-making machines.

But it has quirks. It makes your skin super sensitive to sunlight - you can burn easily. Take it on an empty stomach - food and dairy reduce absorption. And it can permanently stain teeth in kids under 8. That’s why it’s never given to young children.

Side effects include upset stomach (15-25%), dizziness, and photosensitivity. One in five users gets sunburned faster than normal.

Macrolides: The One-Dose Wonder

Azithromycin - often called a “Z-Pack” - is the third most prescribed antibiotic in the U.S. It’s popular because you can take it for just 3-5 days. It treats strep throat, bronchitis, chlamydia, and walking pneumonia.

It binds to bacterial ribosomes and stops protein production. Side effects? Mostly stomach upset - 10-20% of users get nausea or diarrhea. But there’s a bigger risk: it can affect your heart rhythm. A 2022 NEJM study of a million patients found it increases the chance of dangerous heart arrhythmias by over 2 times. That’s why doctors avoid it if you have heart conditions or take other drugs that affect your heartbeat.

High doses can also cause temporary hearing loss. It’s rare, but it happens - especially with prolonged use.

Fluoroquinolones: Powerful, But Risky

Ciprofloxacin and levofloxacin are strong. They’re used for serious infections like kidney infections, pneumonia, and anthrax exposure. They work by cutting bacterial DNA so it can’t copy itself.

But the FDA put a black box warning on them in 2016. Why? They can cause permanent damage. Tendon rupture - especially in the Achilles - happens in 0.1-0.4% of users. Peripheral neuropathy (nerve pain, tingling, numbness) occurs in 0.2-0.5%. And a 2023 JAMA study found a 2.7-fold higher risk of aortic aneurysm - a tear in the main artery.

Because of this, doctors now reserve these for when nothing else works. They’re not for simple sinus infections or bronchitis.

Sulfonamides: For UTIs and Immune Patients

Trimethoprim-sulfamethoxazole (Bactrim or Septra) is common for urinary tract infections and lung infections in people with weakened immune systems, like those with HIV. It blocks bacteria’s ability to make folic acid - something humans get from food, but bacteria must make themselves.

Side effects include nausea, rash, and sun sensitivity. But the big risk? Severe skin reactions. Stevens-Johnson syndrome - a life-threatening blistering condition - occurs in 1-6 cases per million users. It’s rare, but deadly. If you get a fever, blisters, or peeling skin, stop the drug and get help immediately.

Glycopeptides: Last-Resort Weapons

Vancomycin is the antibiotic of last resort. It’s used for MRSA - a deadly bacteria resistant to almost everything else. It’s given intravenously in hospitals, often for serious blood or bone infections.

It works by blocking cell wall building. But it’s harsh. If given too fast, it causes “red man syndrome” - flushing, itching, and low blood pressure. About 5-15% of patients get this. Long-term use can damage kidneys (5-30% risk) and hearing (1-5%).

Because of these risks, vancomycin is kept in the WHO’s RESERVE category - only used when all else fails.

Antibiotic Resistance: Why This Matters

Antibiotics aren’t getting stronger - bacteria are getting smarter. The WHO calls antimicrobial resistance one of the top 10 global health threats. In 2022, it directly caused 1.27 million deaths worldwide.

MRSA, for example, was rare in 2010. Now, over 1% of strains resist vancomycin. That’s up from 0.3%. And we haven’t had a new class of antibiotics since 2000. The pipeline is dry.

The WHO’s AWaRe system helps fix this. It divides antibiotics into three groups:

• ACCESS: First-choice, low resistance risk - like amoxicillin.
• WATCH: Higher resistance risk - like ceftriaxone. Use carefully.
• RESERVE: Last resort - like vancomycin. Only if everything else fails.

Countries using this system have cut inappropriate antibiotic use by 27%. It’s not magic - just better rules.

Common Side Effects You Should Know

Most side effects are mild and go away after you stop the drug. But some are serious. Here’s what to watch for:

• Diarrhea: The #1 complaint. 68% of users report it. Sometimes it’s just upset stomach. Other times, it’s C. diff - a dangerous infection caused by antibiotics killing off good gut bacteria. If you have watery diarrhea, fever, or stomach cramps after starting antibiotics, call your doctor.
• Yeast infections: Especially in women. Antibiotics kill off bacteria that keep yeast in check. Vaginal itching or white discharge? Talk to your pharmacist about over-the-counter treatments.
• Allergic reactions: Rash, hives, swelling, trouble breathing. These can be life-threatening. If you’ve ever had a reaction, tell every doctor you see.
• Drug interactions: Some antibiotics interfere with birth control, blood thinners, or antidepressants. Always check with your pharmacist.

What You Can Do to Stay Safe

You don’t need to avoid antibiotics. But you do need to use them wisely.

• Don’t pressure your doctor for antibiotics. If they say it’s a virus, trust them.
• Take the full course - even if you feel better. Stopping early lets stronger bacteria survive.
• Never share antibiotics. What works for one person might harm another.
• Ask if a narrow-spectrum antibiotic (like amoxicillin) will work instead of a broad one (like ciprofloxacin).
• Report side effects. The more data we have, the safer future prescriptions become.

Antibiotics are powerful tools. But they’re not candy. Misuse doesn’t just hurt you - it hurts everyone. When bacteria become resistant, we all lose.

What’s Next for Antibiotics?

Scientists are racing to find new ones. The FDA approved cefiderocol in 2019 - a new class that works against some of the toughest superbugs. And organizations like CARB-X are investing over $1 billion to fund new antibiotic research.

But the real win? Better prescribing. In the U.S., 85% of hospitals now have antibiotic stewardship programs. These teams review prescriptions to make sure they’re necessary and correct. As a result, inappropriate use has dropped by 35% since 2017.

Still, 30% of outpatient prescriptions have dosing errors. And 45% of bronchitis prescriptions last too long. We’re still overprescribing.

Dr. Helen Boucher, president of the Infectious Diseases Society of America, says we’re one step from returning to the pre-antibiotic era - where a cut on your hand could kill you. The tools exist. What’s missing is discipline.