What Are Antibiotics?

Antibiotics are drugs used for the treatment of infections caused by bacteria. Alexander Fleming discovered the first antibiotic, penicillin, in 1928 while working in a lab at St. Mary’s Hospital, in London. Since then, hundreds of antibiotics have been developed, saving billions of lives worldwide.

There are various types of antibiotics available. Each antibiotic class is effective against certain types of bacteria.

In this article, we’ll explore the different classes of antibiotics, how they work, and when they should or shouldn’t be used.

When To Use Antibiotics

How They Work

Antibiotics work by killing bacteria or stopping them from multiplying. They destroy critical parts of the bacteria, such as the DNA or cell wall. Antibiotics aid the immune system in eliminating bacterial infections but do not directly enhance immune function.

Common and Serious Infections Antibiotics Treat

Antibiotics are used to treat bacterial infections, some being common bacterial infections while others being much more serious requiring more potent and less often used antibiotics.

Common Infections:

Urinary tract infections
Upper respiratory infections such as sinusitis, pharyngitis, and tonsillitis
Ear infections
Skin and soft tissue infections
Sexually transmitted infections such as gonorrhea, chlamydia, and syphilis

Serious Infections:

Lower respiratory tract infections, including pneumonia and bronchitis
Bone and joint infections
Abdominal infections
Bacteremia (bloodstream infections)
Endocarditis (heart infections)
Brain infections such as meningitis and encephalitis
Sepsis

Benefits of Antibiotics

Antibiotics play a crucial role in treating bacterial infections with many benefits:

Eliminate bacterial infections
Reduce symptoms and discomfort
Accelerate recovery time
Prevent the spread of bacterial infections to others
Lower the risk of severe complications

Antibiotics can also be used to prevent bacterial infections, also called antibiotic prophylaxis. For example, antibiotics may be prescribed to people at high risk of infection, such as those undergoing surgery, those with bites or wounds, or those with health problems (cancer, liver disease, HIV) that put them at an increased risk of infection.

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When Not To Use Antibiotics

Choosing Antibiotics for Bacterial Infections

There are various types of antibiotics available to treat bacterial infections. Each antibiotic is effective against certain types of bacteria. For example, ciprofloxacin is effective against Escherichia coli and Pseudomonas aeruginosa.

Healthcare providers select antibiotics based on the likely bacterial cause, patient history, and local antibiotic resistance patterns. You might also have heard of the term “broad-spectrum antibiotics.” These antibiotics are effective against a wide range of Gram-negative and Gram-positive bacteria. Initially, doctors may prescribe a broad-spectrum antibiotic when the exact cause is unknown. Once test results identify the bacteria, they may switch to a more targeted antibiotic.

Other Illnesses That Shouldn’t Be Treated With Antibiotics

Antibiotics do not treat viral infections such as the common cold, flu, or most sore throats (except for bacterial pharyngitis caused by Streptococcus). Similarly, different types of drugs called antifungal agents are used to treat fungal infections.

The Potential Risks of Using Antibiotics When They Aren’t Needed

Taking antibiotics when they aren’t needed will not help your symptoms or cure your infection, but the side effects could still be harmful.

In addition, indiscriminate use of antibiotics for viral infections such as the common cold and flu can lead to the development of antimicrobial resistance or antibiotic resistance. This means bacteria develop defenses against the antibiotics designed to kill them. Once bacteria develop resistance to an antibiotic, different antibiotics, which may be less effective or cause more side effects, are needed to treat infections caused by them. For example, some bacteria produce enzymes called beta-lactamases, which break down beta-lactam antibiotics, rendering them ineffective.

Potential Side Effects of Antibiotics

Common Side Effects

Common side effects of antibiotics include heartburn, stomach pain, nausea, vomiting, diarrhea, loss of appetite, headaches, dizziness, and drowsiness.

Rarely, antibiotics can cause severe allergic reactions, including anaphylactic reactions, requiring emergency medical help. Signs and symptoms of an allergic reaction may include skin rash, hives, itching, difficulty breathing, or swelling of the face or tongue.

An antibiotic overdose can cause serious medical complications. You should call your healthcare provider and get immediate medical help in case of an overdose. The poison control center website also has useful information in case of an antibiotic overdose.

Long-Term Effects of Antibiotics

Long-term antibiotic use can disrupt gut microbiota, increasing the risk of severe diarrhea caused by Clostridioides difficile (C. diff) infections. This can occur due to an imbalance between beneficial and harmful bacteria in the gut after antibiotic therapy.

Antibiotics can also cause more serious side effects, including nerve damage and peripheral neuropathy, tendon rupture, muscle weakness and signs of myasthenia gravis, hypoglycemia (low blood sugar), and severe mood and behavioral changes or worsening of existing mental illness. Other potentially serious side effects may include seizures, liver damage, and heart rhythm abnormalities such as prolonged QT interval.

Managing and Reporting Side Effects

Most of the common side effects of antibiotics are mild and temporary and disappear once you finish treatment. However, it is important to seek immediate medical attention for more serious adverse effects. You can also report any unusual drug reactions to the FDA’s MedWatch program.

Antibiotic List

List of Common Antibiotics: What Are The Top 3 Antibiotics?

The top 3 antibiotics that are most commonly prescribed in the United States are:

Amoxicillin (Amoxil, Sumox, Moxilin, Trimox)
Amoxicillin and clavulanate potassium (Augmentin)
Azithromycin (Zithromax. Z-Pak)

List of Antibiotics Classes

Drug Class	Infections Treated	Examples	Common Side Effects
Aminoglycosides	Reserved to treat serious infections caused by Gram-negative bacteria, for example, to treat severe infections of the urinary tract and abdomen - aminoglycosides are also prescribed for bacteremia and endocarditis (a life-threatening inflammation of the lining of the heart valves	gentamicin tobramycin neomycin streptomycin amikacin	Potential adverse effects of aminoglycosides include hearing loss and kidney injury
Carbapenems	Provide broad-spectrum coverage and can only be given by injection, for example, to treat complicated bacterial infections, including sepsis, pneumonia, and gangrene	meropenem ertapenem doripenem imipenem	Injection site reactions and gastrointestinal disturbance
Cephalosporins	Effective against a wide range of bacteria - oral cephalosporins are used for mild to moderate skin and soft-tissue infections and injectable cephalosporins are effective in treating more serious infections, such as meningitis or infections acquired in the hospital.	cefazolin cefaclor cephalexin cefdinir cefuroxime ceftriaxone	Nausea, vomiting, abdominal pain, decreased appetite
Fluoroquinolones	Provide broad-spectrum coverage and are effective against many different types of bacteria - fluoroquinolone antibiotics are used to treat acute uncomplicated pyelonephritis, moderate to severe urinary tract infections, lower respiratory tract infections such as pneumonia and bronchitis, prostatitis, and sepsis	ciprofloxacin levofloxacin ofloxacin moxifloxacin delafloxacin gemifloxacin	Nausea, vomiting, dizziness, insomnia, headache
Glycopeptides and lipoglycopeptides	Used to treat life-threatening infections caused by multidrug-resistant bacteria	vancomycin telavancin dalbavancin	Headache, nausea, diarrhea
Macrolides	Commonly used to treat respiratory infections such as sinusitis, pharyngitis, tonsillitis, and pneumonia	azithromycin clarithromycin erythromycin	Nausea, vomiting, stomach pain, diarrhea
Penicillins	Widely used to treat Strep throat, respiratory, ear, skin, mouth and throat infections	ampicillin amoxicillin nafcillin dicloxacillin carbenicillin flucloxacillin phenoxymethylpenicillin (penicillin V) penicillin G	Nausea, vomiting, diarrhea, rash, itching, hives
Sulfonamides or “sulfa drugs”	These antibiotics achieve high concentrations in the urine, which makes them effective in treating urinary tract infections	sulfamethoxazole sulfamethizole sulfadiazine sulfasalazine	Skin rash, itching
Tetracyclines	Used to treat acne, respiratory, skin, eye, intestinal, genital, and urinary tract infections	minocycline doxycycline methacycline lymecycline rolitetracycline	Abdominal discomfort, nausea, vomiting, diarrhea, loss of appetite
Oxazolidinones	A relatively newer class of antimicrobial agents - they are effective against a wide range of multidrug-resistant Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci	linezolid tedizolid	Headache, dizziness, nausea, vomiting, diarrhea, stomach pain

Note: There are other classifications of the different types of antibiotics. For example, beta-lactams have a beta-lactam ring in their molecular structure. They include penicillins, cephalosporins, carbapenems, and monobactams.

There are additional classes of antibiotics other than those mentioned above. Some antibiotics, such as clindamycin, mupirocin, metronidazole, fosfomycin, and nitrofurantoin, do not fit into any of the above drug classes. Clindamycin, for example, belongs to a different group called lincosamides.

OTC Antibiotics

Topical Antibiotics

Topical antibiotics are for direct application to the skin. They come in dosage forms such as creams, ointments, gels, solutions, powders, and sprays. Some topical antibiotics are available over-the-counter to treat minor cuts, scrapes, and burns. For example:

Polysporin (bacitracin/polymyxin B)
Neosporin (bacitracin/neomycin/polymyxin B)

Acne Medications

Many over-the-counter acne products contain antibacterial ingredients, such as benzoyl peroxide and azelaic acid.

Frequently Asked Questions

What Are The 3 Main Antibiotics?

Although there are 3 more commonly prescribed antibiotics, including amoxicillin, Augmentin, and azithromycin, there are considered 9 main classes of antibiotics:

Penicillins
Cephalosporins
Aminoglycosides
Tetracyclines
Macrolides
Fluoroquinolones
Carbapenems
Sulfonamides
Glycopeptides

What Are The Big Four Antibiotics?

Some healthcare professionals and researchers refer to macrolides, aminoglycosides, tetracyclines, and beta-lactams as the big four antibiotics.

What Antibiotic Kills All Infections?

There is no single antibiotic that kills all bacteria or cures all infections. Different antibiotics are effective against different types of bacteria. Broad-spectrum antibiotics treat a wide range of infections caused by Gram-positive and Gram-negative bacteria. They can be used to treat infections when the exact causative organism is not known.

Which Is The Strongest Antibiotic?

Vancomycin 3.0 is considered the strongest antibiotic and a drug of last resort. It is used to treat serious infections such as methicillin-resistant Staphylococcus aureus.

Is Ciprofloxacin a Penicillin?

Ciprofloxacin is not a penicillin. It is a broad-spectrum fluoroquinolone antibiotic. It is FDA-approved to treat urinary tract infections, sexually transmitted infections like gonorrhea, prostate gland infections, skin infections, bone and joint infections, gastrointestinal infections, typhoid fever, sinus infections, bronchitis, lower respiratory tract infections, and infectious diseases. It is also prescribed to patients with mixed infections (both gram-positive and gram-negative infections).

Tips For Safe Antibiotic Use

Dosage Forms

Different antibiotics come in different dosage forms. For example, ciprofloxacin is available as immediate-release tablets, extended-release tablets (Cipro XR), and an oral suspension. You should not substitute one type of antibiotic for another without your doctor’s approval even if the active ingredient is the same (for example, ciprofloxacin generic name is the same in different dosage forms but they should not be substituted for one another).

Dosing

Be sure to read your prescription label carefully for the dosing frequency. For example, ciprofloxacin tablets and suspension are usually taken twice a day. However, ciprofloxacin extended-release tablets are taken once daily.

Take your antibiotic at the same time each day. The correct dose and treatment duration depends on the type and severity of the infection. For example, your doctor may prescribe ciprofloxacin 250 mg twice a day for 7-10 days for uncomplicated urinary tract infections or ciprofloxacin 500 mg twice a day for 7-14 days for complicated urinary tract infections. The dose of antibiotics in pediatric patients depends on the child’s weight.

Administration

Do not crush or chew antibiotic tablets - swallow them whole. If you need to split a tablet, break it along the score line. Do not cut, split, crush, or chew extended-release tablets—these should always be swallowed whole.

Check for special instructions regarding liquid medicine. For example, you should shake the bottle of ciprofloxacin oral suspension well before use. Also, you should swallow the ciprofloxacin suspension without chewing the granules it contains.

Make sure you measure the correct dose of liquid medicine using a dose-measuring device, such as a syringe or dosing cup.

Treatment Duration

Follow the directions on the prescription label carefully. Do not take more or less of your antibiotic than prescribed. Also, do not take it for a longer or shorter duration than prescribed. Prolonged therapy with antibiotics can put you at risk of serious adverse effects. Ask your doctor or pharmacist if you don’t understand how to take your antibiotic.

Missed Dose

If you miss a dose of your antibiotic, take it as soon as you remember. However, if it is almost time for the next dose, skip the missed dose and take the next dose according to your regular dosing schedule.

Completing Treatment

You will likely start to feel better after the first few days of antibiotic treatment. Tell your doctor if your symptoms do not improve or become worse. Do not stop taking your antibiotic until you complete the entire course prescribed by your doctor, no matter how much better you are feeling.

Unused Antibiotics

Do not save unused antibiotics for future use. Also, do not take someone else’s antibiotics or share yours with others. Talk to your pharmacist about how to dispose of unused medicine so that children, pets, or others cannot get to it.

Drug Interactions

Your antibiotic can have drug interactions with certain other medicines and natural products. In other words, certain drugs can make your antibiotic less effective or worsen its side effects. Combinations of certain drugs with antibiotics can increase the risk of serious adverse reactions. For example, ciprofloxacin can interact with common over-the-counter pain relievers called nonsteroidal anti-inflammatory drugs (NSAIDs). Taking ciprofloxacin with an NSAID such as Advil (ibuprofen) can lead to severe neurological side effects such as seizures and tremors.

Give your doctor or pharmacist a complete list of your medications, including prescription drugs, over-the-counter medications, herbal remedies, and dietary supplements, to avoid possible drug interactions and adverse events.

Timing of Antibiotic Dose

Your healthcare provider may ask you to take your antibiotic dose away from certain other medicines.

For example, if you are taking antacids (Mylanta, Maalox, Tums, others); calcium, zinc, or iron supplements; phosphate binders like sevelamer (Renvela, Renagel); didanosine (Videx) solution; sucralfate (Carafate); or lanthanum carbonate (Fosrenol), you should take ciprofloxacin at least 2 hours before or 6 hours after these medications. Taking ciprofloxacin at the same time as these medications can make ciprofloxacin a lot less effective.

Food Interactions

For antibiotics that are taken orally (by mouth), your physician will advise you to take it with or without food. They may also ask that you take the medicine away from certain foods. For instance, dairy products, nutritional supplements containing calcium, and calcium-fortified juices can affect ciprofloxacin and should not be taken with the medicine. However, you can take ciprofloxacin with a meal containing these foods and drinks.

Ciprofloxacin can slow caffeine metabolism, leading to increased caffeine effects. It is recommended to limit or avoid caffeinated foods and drinks, such as coffees, teas, colas, chocolate, and energy drinks, while on ciprofloxacin and some other antibiotics. Because of the interaction, the combination of ciprofloxacin and caffeine can lead to insomnia, nervousness, pounding heart, and anxiety.

Your doctor may give additional dietary advice. For example, they may recommend that you take precautions to avoid becoming dehydrated while on an antibiotic such as ciprofloxacin, especially if you have a history of renal impairment or kidney disease. You should drink plenty of water and stay well-hydrated while on this antibiotic.

UV Light

Some antibiotics can increase your skin’s sensitivity to ultraviolet light. For this reason, you need to avoid prolonged sunlight exposure or exposure to sun lamps or tanning beds while on certain antibiotics. Wear protective clothing, sunglasses, and sunscreen when outdoors. Tell your doctor if you develop red, swollen, or blistered skin (like a sunburn).

Antibiotic Resistance

Stopping your antibiotic too soon or skipping doses can lead to incomplete treatment of your infection. It can result in non-clearance of your current infection or antimicrobial resistance. Antibiotic resistance can make future infections harder to treat.

Consult Your Healthcare Professional

Always take antibiotics under the supervision of a healthcare professional. While antibiotics are life-saving drugs that can cure bacterial infections, they can also cause severe adverse effects and drug interactions.

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