Antibiotics: Most Common Types and Their Side Effects

Antibiotics are one of the most important medical breakthroughs in history. Before they existed, a simple cut or sore throat could turn deadly. Today, they save millions of lives every year. But they’re not harmless. Taking antibiotics without understanding how they work-or what they can do to your body-can lead to serious problems. Even common ones like amoxicillin or azithromycin come with risks you might not know about.

How Antibiotics Work (And Why They’re Not for Colds)

Antibiotics only work on bacterial infections. They don’t help with viruses like the flu, colds, or most sore throats. Using them anyway doesn’t speed up recovery-it just increases your risk of side effects and contributes to antibiotic resistance. That’s when bacteria evolve to survive the drugs meant to kill them. The World Health Organization calls this one of the top 10 global health threats.

There are two main ways antibiotics work: some kill bacteria outright (bactericidal), while others stop them from multiplying (bacteriostatic). The difference matters because it affects how long you need to take them and what kind of infection they’re best for. But in practice, doctors pick based on the infection type, not just the mechanism.

The Seven Most Common Antibiotic Classes

Not all antibiotics are the same. They fall into groups based on their chemical structure and how they attack bacteria. Here are the seven most commonly prescribed classes in the U.S. today, along with real-world examples and what they’re used for.

• Penicillins - Includes amoxicillin, ampicillin, penicillin V. These were the first antibiotics ever used. They disrupt the bacterial cell wall. Amoxicillin alone accounts for nearly 120 million U.S. prescriptions each year. It’s the go-to for ear infections, sinus infections, and strep throat.
• Cephalosporins - Includes cephalexin, ceftriaxone, cefuroxime. Often used when someone says they’re allergic to penicillin. But here’s the catch: only 1-3% of people with penicillin allergies actually react to cephalosporins. Cephalexin is one of the top 5 most prescribed antibiotics in the U.S., mostly for skin infections and urinary tract infections.
• Tetracyclines - Includes doxycycline and tetracycline. These stop bacteria from making proteins. Doxycycline is widely used for acne, Lyme disease, and some respiratory infections. It’s cheap, effective, and taken once daily. But it’s not for kids under 8 or pregnant women-it can permanently stain developing teeth.
• Macrolides - Includes azithromycin (Z-Pak), clarithromycin. These are popular for people with penicillin allergies. Azithromycin is the third most prescribed antibiotic in the U.S., often given as a 5-day course. It’s used for pneumonia, bronchitis, and STIs like chlamydia.
• Fluoroquinolones - Includes ciprofloxacin, levofloxacin. These are powerful but risky. They interfere with bacterial DNA replication. Once a go-to for UTIs and sinus infections, they’re now reserved for serious cases because of dangerous side effects like tendon rupture and nerve damage. The FDA has issued black box warnings for these drugs.
• Sulfonamides - Includes trimethoprim-sulfamethoxazole (Bactrim). This combo is used for urinary tract infections and to prevent pneumonia in people with weakened immune systems. It’s affordable and effective, but can cause serious skin reactions in rare cases.
• Glycopeptides - Includes vancomycin. This is a last-resort drug for infections like MRSA. It’s usually given in the hospital through an IV. Vancomycin is so important that the WHO classifies it as a “reserve” antibiotic-only to be used when nothing else works.

Side Effects: What Most People Experience

Almost everyone who takes antibiotics gets at least one side effect. The most common? Gastrointestinal issues. Nausea, stomach pain, and diarrhea are so frequent that they’re considered normal-not rare. In fact, 15-20% of people taking penicillins get nausea. Up to 10% get diarrhea. And for some, it’s worse.

Antibiotics kill off good bacteria in your gut along with bad ones. This imbalance can lead to C. diff infection-a serious, sometimes life-threatening diarrhea caused by an overgrowth of harmful bacteria. It’s more common in older adults and people who’ve taken multiple courses of antibiotics.

Yeast infections are another common side effect, especially in women. Antibiotics reduce the good bacteria that keep yeast in check. A vaginal yeast infection after a course of antibiotics isn’t rare-it’s expected in about 2-8% of women who take them.

Serious Side Effects You Can’t Ignore

Some side effects aren’t just uncomfortable-they can be permanent or deadly.

• Penicillin allergy - About 10% of Americans say they’re allergic to penicillin. But studies show 90% of them aren’t truly allergic. Many outgrow it, or were misdiagnosed after a rash as a child. If you think you’re allergic, ask for an allergy test. It could open up safer, more effective treatment options.
• Photosensitivity - Doxycycline makes your skin super sensitive to sunlight. You can get a bad sunburn after just 10 minutes outside. Always wear sunscreen and avoid tanning beds while taking it.
• Tendon damage - Fluoroquinolones like ciprofloxacin can cause tendon rupture, especially in the Achilles tendon. The risk goes up if you’re over 60, on steroids, or have kidney disease. The FDA warns against using these for simple infections like sinusitis or bronchitis.
• Nerve damage - Fluoroquinolones can cause permanent peripheral neuropathy-numbness, tingling, or burning in the hands and feet. Symptoms can start within hours and last for years.
• Heart rhythm problems - Azithromycin can slightly lengthen the QT interval on an EKG. For most people, this isn’t a problem. But if you have heart disease, take certain heart medications, or have low potassium, this can lead to dangerous arrhythmias.
• Red man syndrome - This happens with vancomycin if it’s given too fast. It causes flushing, itching, and redness across the upper body. It’s not an allergy-it’s a reaction to the infusion speed. Slowing down the drip prevents it.

Why Antibiotic Resistance Is a Silent Crisis

Every time you take an antibiotic, you’re helping bacteria learn how to survive it. Overuse is the main driver. In the U.S., doctors prescribe antibiotics for viral infections about 30% of the time when they shouldn’t. That’s not just ineffective-it’s dangerous.

MRSA (methicillin-resistant Staphylococcus aureus) used to be rare. Now it’s common in hospitals and even gyms and schools. Vancomycin used to cure it. Now, some strains are resistant to vancomycin too. The CDC reports vancomycin-resistant MRSA jumped from 0.3% in 2010 to 1.2% in 2022.

The WHO’s AWaRe system helps doctors choose wisely: ACCESS (safe first-choice drugs), WATCH (use carefully), and RESERVE (only for last-resort cases). Amoxicillin is ACCESS. Ceftriaxone is WATCH. Vancomycin is RESERVE. Countries using this system cut inappropriate use by 27%.

What You Should Do (And Not Do)

Here’s how to use antibiotics safely:

1. Don’t demand antibiotics - If your doctor says it’s a virus, trust them. Most colds and coughs don’t need them.
2. Take them exactly as prescribed - Even if you feel better, finish the full course. Stopping early lets the toughest bacteria survive and multiply.
3. Never share antibiotics - A drug that worked for your friend might not work for you-and could make your infection worse.
4. Ask about alternatives - If you’re prescribed a broad-spectrum drug like ciprofloxacin, ask if a narrower option exists. Narrow-spectrum drugs target specific bacteria and are less likely to cause resistance.
5. Report side effects - If you get a rash, severe diarrhea, or unusual pain after starting an antibiotic, tell your doctor. You might be part of the data that helps improve prescribing.

What’s Next for Antibiotics?

The pipeline for new antibiotics is dry. Only two new classes have been approved since 2000. Pharma companies don’t invest much because antibiotics aren’t profitable-they’re taken for short periods, and doctors are told to use them sparingly.

But there’s hope. The U.S. and UK have pledged $1 billion to fund new antibiotic research through CARB-X. New drugs like cefiderocol are showing promise against drug-resistant infections. Hospitals are now required to have antibiotic stewardship programs, which have cut unnecessary use by 35%.

The real solution? Better diagnosis, faster tests to identify bacteria, and smarter prescribing. Until then, the most powerful antibiotic you have is knowing when to use-and when not to use-one.