What Are Antibiotics?

By Frieda Wiley, PharmD, CGP, RPh

Medically Reviewed by Rosalyn Carson-DeWitt, MD

Last Updated: 10/19/2015

These powerful drugs treat many bacterial infections, but experts fear they may be over-prescribed.

Antibiotics are a group of prescription drugs used to treat a wide variety of infections caused by bacteria.

They aren't effective against fungal or viral infections, such as a cold or flu. There are other drug classes designed to treat those infections.

There are many different classes and subclasses of antibiotics, including:

  • Penicillins, such as Amoxil and Augmentin (amoxicillin) and Unasyn (ampicillin)
  • Cephalosporins, including cefdinir, Rocephin (ceftriaxone), and Keflex (cephalexin)
  • Fluoroquinolones ("quinolones"), like Levaquin (levofloxacin), Cipro (ciprofloxacin), and Avelox (moxifloxacin)
  • Macrolides, such as Zithromax or Z-pak (azithromycin); Ery-Tab, Akne-Mycin, E.E.S., Eryc, and Pediamycin (erythromycin); and Cleocin, Cleocin T, ClindaGel, and Clinda-Derm (clindamycin)
  • Tetracyclines, including tetracycline and Vibramycin (doxycycline)
  • Aminoglycosides, like amikacin; Genoptic and Gentak (gentamicin); Aktob, Bethkis, Kitabis Pak, Tobi, Tobi Podhaler, Tobradex, and Tobrex (tobramycin); and Neo-Fradin (neomycin)
  • Sulfonamides (sulfa drugs), such as Septra and Bactrim (sulfamethoxazole with trimethoprim)

Not all antibiotics work the same way to fight infections.

Some antibiotics are bactericidal, meaning that they kill bacteria, while others are bacteriostatic, meaning that they prevent bacteria from reproducing.

For example, penicillins, cephalosporins, and aminoglycosides are bactericidal, while macrolides, tetracyclines, and sulfonamides are bacteriostatic.

Warnings and Precautions

Since allergies to certain antibiotics like penicillins and sulfa drugs are common, it's always a good idea to find out what class or subclass your prescribed antibiotic is in.

Unless your doctor tells you otherwise, be sure to finish taking all of the antibiotics you were prescribed — even if you start to feel better.

This is extremely important to make sure the infection goes away completely.

If you fail to finish the entire course of therapy, the infection may still be present, and symptoms may return.

Then, if the infection comes back, it will most likely be worse and more difficult to treat.

It's also possible that the antibiotics your doctor originally prescribed may not work as well — or at all — because the bacteria may have become resistant to the original drug.

Antibiotic Resistance

Bacterial resistance to antibiotics is a widespread and growing problem.

In fact, some antibiotics that were effective against certain infections just a few years ago are no longer useful.

Overuse of antibiotics contributes to bacterial resistance. Resistance occurs when bacteria develop the ability to continue growing, despite the presence of a particular antibiotic.

The Centers for Disease Control and Prevention (CDC) has stated that "up to 50 percent of the time, antibiotics are not optimally prescribed, often done so when not needed, [or with] incorrect dosing or duration."

And most of the antibiotics used in the United States (as many as 75 percent or more) are not provided to sick people, but are given to farm animals to prevent disease and encourage growth.

Resistance may occur when antibiotics are taken for a condition that isn't caused by bacteria, such as:

  • Cold
  • Flu (influenza)
  • Viral gastroenteritis ("stomach flu")
  • Most coughs
  • Most sore throats

There's no benefit from taking antibiotics for these infections, and doing so may put you and others at risk for antibiotic-resistant infections.

General Side Effects

Although most people can take antibiotics successfully, possible side effects include:

  • Sun sensitivity
  • Stomach upset
  • Diarrhea
  • Vaginal yeast infection
  • Metallic taste in the mouth
  • Allergy to the drug, resulting in rash or hives
  • Severe, potentially life-threatening swelling due to drug sensitivity, called anaphylaxis

However, in more severe cases, some antibiotics — like Augmentin (amoxicillin and clavulanate) and clindamycin — may cause a very severe form of diarrhea, characterized by frequent episodes of loose, watery stools with a strong odor that is much more unpleasant than normal.

If this happens, stop taking the antibiotic immediately and contact your doctor right away. You may have to take a different antibiotic.

Certain antibiotics may cause side effects such as tooth discoloration in baby teeth, hearing loss, or kidney problems.

Drug Interactions

Antibiotics, like most drugs, can interact with other drugs. Some, like Zithromax or Z-Pak (azithromycin), generally don't have many drug interactions.

But other antibiotics, like fluoroquinolones and tetracyclines, don't work as well if you take them at the same time as calcium, iron, antacids like Tums or Maalox, or foods such as milk, cheese, or nuts.

If you're taking medications or supplements that contain calcium or iron, or eating dairy foods or nuts, try to take your antibiotics a few hours before consuming any of these items.

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