Antibiotics; Types, Uses, Side Effects, Drug Interactions

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Antibiotics also called an antibacterial type of antimicrobial drug used in the treatment and prevention of bacterial infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the common cold or influenza; drugs which inhibit viruses are termed antiviral drugs or antivirals rather than antibiotics.

You’ve most likely taken an antibiotic at least once in your lifetime. From treatments for painful strep throat or ear infections as a child, to burning urinary tract infections or itchy skin infections as an adult, antibiotics are one of the most highly utilized and important medication classes we have in medicine.

Understanding the vast world of antibiotics and anti-infectives is no easy task. Anti-infectives are a large class of drugs that cover a broad range of infections, including fungal, viral, bacterial, and even protozoal infections. Athletes foot? That’s a common fungal infection. HIV? Antiviral medications are always needed. Bladder infection? Yes, that may need a common antibiotic. And head lice? A topical anti-parasitic can alleviate the itching. There is no one type of antibiotic that cures every infection. Antibioticsspecifically treat infections caused by bacteria, such as Staph., Strep., or E. coli., and either kill the bacteria (bactericidal) or keep it from reproducing and growing (bacteriostatic). Antibiotics do not work against any viral infection.

When To Use Antibiotics

Antibiotics are specific for the type of bacteria being treated and, in general, cannot be interchanged from one infection to another. When antibiotics are used correctly, they are usually safe with few side effects.

However, as with most drugs, antibiotics can lead to side effects that may range from being a nuisance to serious or life-threatening. In infants and the elderly, in patients with kidney or liver disease, in pregnant or breastfeeding women, and in many other patient groups antibiotic doses may need to be adjusted based upon the specific characteristics of the patient, like kidney or liver function, weight, or age. Drug interactions can also be common with antibiotics. Health care providers are able to assess each patient individually to determine the correct antibiotic and dose.

When NOT To Use Antibiotics

Antibiotics are not the correct choice for all infections. For example, most sore throats, cough and colds, flu or acute sinusitis are viral in origin (not bacterial) and do not need an antibiotic. These viral infections are “self-limiting”, meaning that your own immune system will usually kick in and fight the virus off. In fact, using antibiotics for viral infections can increase the risk for antibiotic resistance, lower the options for future treatments if an antibiotic is needed, and put a patient at risk for side effects and extra cost due to unnecessary drug treatment.

Antibiotic-resistant bacteria cannot be fully inhibited or killed by an antibiotic, even though the antibiotic may have worked effectively before the resistance occurred. Don’t share your antibiotic or take medicine that was prescribed for someone else, and don’t save an antibiotic to use the next time you get sick.

Antibiotics Classification

Although there are several systems for classification of antibiotics, the most useful is based on chemical structure. Antibiotics within a structural class will generally have similar patterns of effectiveness, toxicity, and allergic potential.

The main classes of antibiotics are:

  • Beta-Lactams
    • Penicillins
    • Cephalosporins
  • Macrolides
  • Fluoroquinolones
  • Tetracyclines
  • Aminoglycosides

Most commonly used types of antibiotics are: Aminoglycosides, Penicillins, Fluoroquinolones, Cephalosporins, Macrolides, and Tetracyclines. While each class is composed of multiple drugs, each drug is unique in some way.


The penicillins are the oldest class of antibiotics. Penicillins are bicyclic penam compounds and share their chemical structure with the cephalosporins.

Penicillins are generally bactericidal, inhibiting the formation of the bacterial cell wall. Penicillins are used to treat skin infections, dental infections, ear infections, respiratory tract infections, urinary tract infections, and gonorrhea.

There are four types of penicillins:

  • The natural penicillins are based on the original penicillin-G structure. Natural penicillins are active against gram-positive streptococci, staphylococci, and some gram-negative bacteria such as meningococcus.
  • Penicillinase-resistant penicillins (e.g. methicillin, oxacillin) are active against beta-lactamase producing bacteria, that inactivates most penicillin antibiotics.
  • Aminopenicillins such as ampicillin and amoxicillin are effective against a wider range of bacteria and have a better oral absorption.
  • Extended-spectrum penicillins (e.g. mezlocillin, piperacillin, ticarcillin).
Penicillins side effects

Penicillins are generally very safe drugs with minimum toxicity. Their most common side effect is diarrhea. Nausea, vomiting, and upset stomach are also common. In rare cases penicillins can cause immediate or delayed allergic reactions which manifest as skin rashes, fever, angioedema, and anaphylactic shock. Severe hypersensitivity reactions are more common after injections than after oral formulations.

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Very high doses of penicillins, especially in patients with renal impairment, may cause convulsions.

All penicillins are classed as Pregnancy category B.


Cephalosporins have a mechanism of action identical to that of the penicillins. However, the basic chemical structure of the penicillins and cephalosporins differs in other respects, resulting in different spectrum of antibacterial activity. Like the penicillins, cephalosporins have a beta-lactam ring structure that interferes with synthesis of the bacterial cell wall and so are bactericidal. Cephalosporins are derived from cephalosporin C which is produced from Cephalosporium acremonium.

Cephalosporins are used to treat pneumonia, strep throat, staph infections, tonsillitis, bronchitis, otitis media, various types of skin infections, gonorrhea, urinary tract infections Cephalosporin antibiotics are also commonly used for surgical prophylaxis. Cephalexin can also be used to treat bone infections.

Cephalosporins are extremely diverse class of antibiotics, they are grouped into “generations” by their antimicrobial properties. Each newer generation has a broader spectrum of activity than the one before.

  • The first generation cephalosporins have quite similar spectrums of activity. They have excellent coverage against most gram-positive pathogens but variable to poor coverage against most gram negative pathogens. The first generation includes:
    • cephalothin
    • cefazolin
    • cephapirin
    • cephradine
    • cephalexin
    • cefadroxil
  • The second generation cephalosporins have expanded gram negative spectrum in addition to the gram positive spectrum of the first generation cephalosporins. Cefoxitin and cefotetan have good activity against Bacteroides fragilis. Enough variation exists between the second generation cephalosporins in regard to their spectrums of activity against most species of gram negative bacteria, that susceptibility testing is generally required to determine sensitivity. The second generation includes:
    • cefaclor
    • cefamandole
    • cefonicid
    • ceforanide
    • cefuroxime
  • The third generation cephalosporins have much expanded gram negative activity. However, some members of this group have decreased activity against gram-positive organisms. They have the advantage of convenient dosage regimen, but they are expensive. The third generation includes:
    • cefcapene
    • cefdaloxime
    • cefditoren
    • cefetamet
    • cefixime
    • cefmenoxime
    • cefodizime
    • cefoperazone
    • cefotaxime
    • cefpimizole
    • cefpodoxime
    • ceftibuten
    • ceftriaxone
  • The fourth generation cephalosporins are extended-spectrum agents with similar activity against gram-positive organisms as first-generation cephalosporins. They also have a greater resistance to beta-lactamases. Many fourth generation cephalosporins can cross blood brain barrier and are effective in meningitis. The fourth generation includes:
    • cefclidine
    • cefepime
    • cefluprenam
    • cefozopran
    • cefpirome
    • cefquinome
Cephalosporins side effects

Cephalosporins are the remarkably safe class of antibiotics and usually cause few adverse effects. Common side effects include diarrhea, nausea, mild stomach cramps or upset. Approximately 5–10% of patients with allergic hypersensitivity to penicillins will also have cross-reactivity with cephalosporins. Thus, cephalosporin antibiotics are contraindicated in people with a history of allergic reactions (urticaria, anaphylaxis, interstitial nephritis, etc) to penicillins or cephalosporins.

Hematologic toxicity

Thrombocytopenia, neutropenia, abnormalities of platelet function and coagulation have been reported with certain cephalosporins.

Cephalosporin antibiotics are classed as Pregnancy category B.


Fluoroquinolones (fluoridated quinolones) are the newest class of antibiotics. Their generic name often contains the root “floxacin”. They are synthetic compounds, and are not derived from bacteria.

The earliest first-generation medications are referred as quinolones, and newer generations as fluoroquinolones. The older quinolones are not well absorbed and are used to treat mostly urinary tract infections. The fluoroquinolones are broad-spectrum agents with excellent oral bioavailability. Because of their high absorption fluoroquinolones can be administered not only intravenously but orally as well.

Fluoroquinolones are used to treat urinary tract infections, skin infections, and respiratory infections (such as sinusitis, pneumonia, bronchitis), pulmonary infections in cystic fibrosis.

Fluoroquinolones are bactericidal and kill bacteria by inhibiting bacterial enzyme DNA gyrase.

Fluoroquinolone family includes:

  • clinafloxacin
  • ciprofloxacin
  • gatifloxacin
  • levofloxacin
  • lomefloxacin
  • ofloxacin
  • sparfloxacin
  • trovafloxacin
Fluoroquinolones side effects

Fluoroquinolones are generally well tolerated and have acceptable level of safety. The most common side effects include nausea, vomiting, diarrhea, abdominal pain. More serious but less common side effects are central nervous system disturbances (headache, confusion, dizziness, tremor), phototoxicity (more common with lomefloxacin and sparfloxacin), prolongation of the QT interval7, tendinopathy and tendon rupture, and convulsions.

Fluoroquinolones should be avoided when possible in pregnant women and children.


Tetracyclines are an old class of antibiotics. They got their name for their chemical structure which contains four hexagonal rings. Tetracyclines are derived from a species of Streptomyces bacteria.

Tetracycline antibiotics are bacteriostatic agents and work by inhibiting the bacterial protein synthesis via interaction with the 30S subunit of the bacterial ribosome. Tetracyclines are effective against a wide variety of microorganisms, including spirochetes, atypical bacteria, rickettsia, and amebic parasites.

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Current applications of tetracyclines include treatment of peptic ulcer disease as part of a multi-drug regimen, infections of the respiratory tract, cholera, Rocky Mountain spotted fever, Lyme disease, typhus, prophylaxis of traveler’s diarrhea, malaria prophylaxis. Their most common current use is in the treatment of acne vulgaris and rosacea.

Tetracycline antibiotics are:

  • doxycycline
  • minocycline
  • oxytetracycline
  • tetracycline
Tetracyclines side effects

Common side effects associated with tetracyclines include stomach cramps, diarrhea, nausea, vomiting, esophageal ulceration, sore mouth or tongue. Tetracyclines can cause skin photosensitivity, which increases the risk of sunburn under exposure to UV light. This may be of particular importance for those intending to take on holidays long-term doxycycline as a malaria prophylaxis.

Rarely, tetracyclines may cause allergic reactions. Very rarely severe headache and vision problems may be signs of dangerous secondary intracranial hypertension.

Tetracycline antibiotics should not be used in children under the age of 8, and specifically during periods of tooth development. Tetracyclines are classed as pregnancy category D. Tetracyclines may cause the gray to yellow discoloration of actively forming teeth and deposition in growing bones.


The macrolide antibiotics owe their name to a macrocyclic lactone ring in their chemical structure. They are derived from Streptomyces bacteria.

The macrolides target bacterial ribosomes and prevent protein production and are mainly bacteriostatic agents.

Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Newer members of the group, azithromycin and clarithyromycin, are particularly useful for their excellent lung penetration. Macrolide antibiotics are used to treat respiratory tract infections (such as pharyngitis, sinusitis, and bronchitis), genital, gastrointestinal tract, and skin infections.

Macrolide antibiotics are:

  • azithromycin
  • clarithromycin
  • dirithromycin
  • erythromycin
  • roxithromycin
  • troleandomycin
Macrolides side effects

Macrolides are usually tolerated quite well. Most common adverse effects include nausea, vomiting, abdominal discomfort, and diarrhea. They have been rarely associated with reversible deafness and allergic reactions (including angioedema, anaphylaxis, and dermatologic reactions). Oral erythromycin may be highly irritating to the stomach and when given by injection may cause severe thrombophlebitis. Macrolide antibiotics should be used with caution in patients with liver dysfunction.

Pregnancy category B: Azithromycin, erythromycin.
Pregnancy category C: Clarithromycin, dirithromycin, troleandomycin.


Aminoglycosides are derived from various species of Streptomyces.

In 1943, Selman Waksman, together with his co-workers, discovered that a bacterium Streptomyces griseus produced an antibiotic substance which they named “streptomycin.” Selman Waksman was awarded the Nobel Prize in Physiology or Medicine in 1952 for his discovery of streptomycin.

The aminoglycosides are bactericidal and work by binding to the 30S subunit of the bacterial ribosome, thus stopping bacteria from making proteins.

Aminoglycoside antibiotics are used to treat infections caused by gram-negative bacteria. Aminoglycosides may be used in combination with with penicillins or cephalosporins to ensure better antimicrobial coverage. Aminoglycosides work quite well, but bacteria can become resistant to them. Since aminoglycosides are broken down easily in the stomach, they can’t be given by mouth and must be injected. Generally, aminoglycosides are given for short time periods.

Aminoglycoside grope includes:

  • amikacin
  • gentamicin
  • kanamycin
  • neomycin
  • streptomycin
  • tobramycin

List of Antibiotic Classes in details(Types of Antibiotics)

Most antibiotics fall into their individual antibiotic classes. An antibiotic class is a grouping of different drugs that have similar chemical and pharmacologic properties. Their chemical structures may look comparable, and drugs within the same class may kill the same or related bacteria.

However, it is important not to use an antibiotic for an infection unless your doctor specifically prescribes it, even if it’s in the same class as another drug you were previously prescribed. Antibiotics are specific for the kind of bacteria they kill. Plus, you would need a full treatment regimen to effectively cure your infection, so don’t use or give away leftover


Another name for this class is the beta-lactam antibiotics, referring to their structural formula. The penicillin class contains five groups of antibiotics: aminopenicillins, antipseudomonal penicillins, beta-lactamase inhibitors, natural penicillins, and the penicillinase-resistant penicillins. Common antibiotics in the penicillin class include:

  • penicillin V potassium
  • amoxicillin
  • amoxicillin/clavulanate (Augmentin)


Tetracyclines are broad-spectrum against many bacteria and treat conditions such as acne, urinary tract infections (UTIs), intestinal tract infections, eye infections, sexually transmitted diseases, periodontitis (gum disease), and other bacterial infections. The tetracycline class contains well-known drugs such as:

  • doxycycline
  • tetracycline
  • minocycline


There are five generations of cephalosporins, with increasing expanded coverage to include gram-negative infections. Cephalosporins treat many infections, including strep throat, ear infections, urinary tract infections, skin infections, and meningitis. The fifth generation cephalosporin ceftaroline (Teflaro) is active against methicillin-resistant Staphylococcus aureus (MRSA). You’ve probably heard of common medications in this class, like:

  • cefuroxime (Ceftin)
  • ceftriaxone (Rocephin)
  • Cefdinir (Omnicef)
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The quinolones, also known as the fluoroquinolones, are a synthetic, bactericidal antibacterial class with a broad-spectrum of activity. The quinolones can be used for difficult-to-treat urinary tract infections when other options are aren’t effective, hospital-acquired pneumonia, bacterial prostatitis, and even anthrax or plague. The FDA issued a strong warning about this class in 2016. Familiar names in the fluoroquinolone class include:

  • ciprofloxacin (Cipro)
  • levofloxacin (Levaquin)
  • moxifloxacin (Avelox)


This class has activity against gram-positive aerobes and anaerobes (bacteria that can live without oxygen), as well as some gram-negative anaerobes. The lincomycin derivatives may be used to treat serious infections like pelvic inflammatory disease, intra-abdominal infections, lower respiratory tract infections, and bone and joint infections. These drugs include:

  • clindamycin (Cleocin)
  • lincomycin (Lincocin)


The macrolides can be use to treat community-acquired pneumonia, pertussis (whooping cough), or for uncomplicated skin infections, among other susceptible infections. Ketolides are a newer generation of antibiotic developed to overcome macrolide bacterial resistance. Frequently prescribed macrolides are


Sulfonamides are effective against some gram-positive and many gram-negative bacteria, but resistance is widespread. Common uses for sulfonamides include UTIs, treatment or prevention of pneumocystis pneumonia, or ear infections (otitis media). Familiar names include:

  • sulfamethoxazole-trimethoprim
  • sulfasalazine
  • sulfisoxazole

Glycopeptide Antibiotics

Members of this group may be used for treating methicillin-resistant staphylococcus aureus (MRSA) infections, complicated skin infections, C. difficile-associated diarrhea, and enterococcal infections such as endocarditis which are resistant to beta-lactams and other antibiotics. Common drug names include:

  • dalbavancin
  • oritavancin
  • telavancin
  • vancomycin


Aminoglycosides inhibit bacterial synthesis by binding to the 30S ribosome and act rapidly as bactericidal antibiotics (killing the bacteria). These drugs are usually given intravenously (in a vein through a needle). Common examples in this class are:

  • gentamicin
  • tobramycin
  • amikacin


These injectable beta-lactam antibiotics have a wide spectrum of bacteria-killing power and may be used for moderate to life-threatening bacterial infections like stomach infections, pneumonias, kidney infections, multidrug-resistant hospital-acquired infections and many other types of serious bacterial illnesses. Members of this class include:

  • imipenem/cilastatin
  • meropenem
  • doripenem
  • ertapenem

Over the Counter Antibiotics

Over-the-counter (OTC) oral antibiotics are not approved in the U.S. A bacterial infection is best treated with a prescription antibiotic that is specific for the type of bacteria causing the infection. This will increase the chances that the infection is cured and help to prevent antibiotic resistance. In addition, a lab culture may need to be performed to pinpoint the bacteria and to help select the best antibiotic. Taking the wrong antibiotic — or not enough — may worsen the infection and prevent the antibiotic from working the next time.

There are a few OTC topical antibiotics that can be used on the skin. Some products treat or prevent minor cuts, scrapes or burn on the skin that may get infected with bacteria. These are available in creams, ointments, and even sprays.

Common OTC topical antibiotics

  • Neosporin, Medi-Quik (bacitracin/neomycin/polymyxin B)
  • Polysporin (bacitracin/polymyxin)
  • Triple antibiotic, generic (bacitracin/neomycin/polymyxin B)
  • Neosporin Plus (neomycin/polymyxin/pramoxine)

There are some OTC antibacterials for acne sufferers, too. They contain the antibacterial benzoyl peroxide, which also has mild drying effect for acne. Many products are found on the pharmacy shelves as gels, lotions, solutions, foams, cleaning pads, and even facial scrubs.

Common OTC antibacterials for acne

  • Clearskin
  • Oxy-10
  • Proactiv

Trending Antibiotic Articles

Patients frequently have questions about specific topics with antibiotics. Here are some articles that address common questions you may have about antibiotics.

  • Trying to make sense of the side effects noted on your antibiotic prescription medication guide? Read the Common Side Effects, Allergies and Reactions to Antibiotics article to understand the meanings.
  • Don’t understand what is meant by antibiotic resistance? Check out our Antibiotic Resistanceprimer and statistics on inappropriate antibiotic use.
  • Should you be concerned about taking Cipro
  • Wondering if you can still have that evening cocktail with your antibiotic? Scan the Antibiotics and Drinking Alcohol information to learn if you should skip it or not.
  • Think You’re Allergic to Penicillin? Maybe Not, Check out why here.
  • FDA clears the first test to help find antibiotic-resistant bacteria.


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