Antibiotics Side Effects

Antibiotics are one of the greatest discoveries of modern medicine and they have forever changed its face. The have significantly reduced the threat of bacterial infections and they are one of the most commonly prescribed drugs worldwide. However, antibiotic therapy also has certain severe side effects and complications.


First, I would like to show a short the list of most commonly used antibiotics.

Beta-lactam antibiotics

a) Penicillin antibiotics

These include antibiotics such as benzylpenicillin, aminopenicillins and oxacillin. Penicillin antibiotics are generally useful against streptococci, while oxacillin is a special penicillin, which is efficient against staphylococci.

b) Cephalosporins

These are often used antibiotics, we know three main generations of cephalosporines. First generation includes substance known as cefadroxil, second generation includes cefprozil and cefuroxime and third generation includes for example substance known as cefoperazone.

c) Carbapenems

Antibiotics in this group are usually not served commonly. They represent a "last line of defense" and they are used in very serious infectious conditions. Representative worth mentioning is a substance known as meropenem.


Probably the most used compound is doxycycline. It may be used in atypical pneumonias and in Lyme disease.


This group of antibiotics has usually good effect in bone infections such as osteomyelitis. The most widely used substance is clindamycin.


Macrolides are great against atypical pneumonias. They are generally effective against intracellular pathogens such as chlamydial infections. Widely prescribed active substances such as erythromycin, clarithromycin, azithromycin and roxithromycin belong among macrolides.


This group of antibiotics includes compounds such as amikacin and gentamicin. They are usually effective, but we should be cautious by their nephrotoxicity (kidney damage) – see below.


These antibiotics are frequently prescribed in urinary infections, infections of gallbladder and infections of bile ducts. It is important to mention active substance like ciprofloxacin, norfloxacin and ofloxacin.


Phenicols include highly efficient, but virtually unused (because of its serious possible side effects) compound chloramphenicol.


These antibiotics are usually used in urinary tract infections. Prescribed active substances include nitrofurantoin, nifuroxazide and nifuratel.


The most used compound of this class is metronidazole. It is prescribed in certain bacterial infections of the gastrointestinal tract.


Today, these drugs are mainly used in urology. The preparations are typically combinations of sulfonamide compound trimethoprim with sulfamethoxazole.


This relatively new and effective group of antibiotics includes teicoplanin and vancomycin. Teicoplanin is rather used in serious infections, while vancomycin is great in therapy of pseudomembranous colitis (see below).

Side effects and disadvantages of antibiotics


Diarrhea is a typical side effect of antibiotic therapy. This is mainly due to the fact that the intestinal flora gets destroyed. Intestinal flora consists of beneficial bacteria that live with us in tight symbiosis. They facilitate proper digestion and even produce some important substances such as vitamin K. Unfortunately, these bacteria are also destroyed by antibiotics and this may cause multiplication of various harmful microorganisms, causing infectious diarrhea. Diarrhea may not sound like a big complication, but it can cause serious dehydration in old and weakened patients. The situation may be prevented and treated by probiotics. Probiotics are preparations with beneficial intestinal microorganisms that are aimed to normalize the intestinal flora.

Pseudomembranous colitis

This is a serious condition, which could be considered as a complication of the previous problem. In some cases, the disruption of intestinal microorganisms may cause proliferation of an aggressive bacterium known as clostridium difficile. These bacteria produce a toxin, which damages the intestinal wall, causing so-called pseudomembranous enterocolitis. Virtually any antibiotics can cause this situation; the risk is significantly high for example in lincosamides (see the antibiotic overview above). The disease usually occurs a few days after starting treatment with antibiotics. It manifests with abdominal pain, abdominal cramps, fever and diarrhea. In the worst case, the disease may cause complicated toxic megacolon. The disease is paradoxically treated with antibiotics, but we must choose antibiotics effective against clostridium bacteria. Best choices are vancomycin and metronidazole. The therapy may be supported by probiotics. Some studies have reported efficiency of the so-called stool transplant.

Resistance to antibiotics

Irresponsible prescription of antibiotics and their overuse leads to occurrence of bacterial resistance to antibiotics, making the antibiotics useless. You can find more information in the related article.

Damage of the growth cartilages

This is an unpleasant complication, which is typical for fluoroquinolone antibiotics. Growth cartilage occurs in growth plates of bones of children and adolescents. These growth plates allow the bone to lengthen. At the end of adolescence, the growth plates close and bone growth is stopped. Certain antibiotics can cause premature closure of the growth plates, thus stopping the growth of a child, or causing various bone deformations. Therefore, these antibiotics may be prescribed only to adults and never to children, or women, who are pregnant, or breastfeeding.


The antibiotics may cause allergic reactions. Typically, it is just an allergic rash. Some antibiotics like tetracyclines may cause allergic photodermatitis, i.e. allergic rash that deteriorates in exposure of skin to sunlight. Theoretically, even more severe allergic reactions may occur, including the anaphylactic shock.

Mononucleosis-related side effect

When you administer amoxicillin antibiotic to a person with mononucleosis, it frequently causes reddish rash. This can simply happen, because the mononucleosis has very similar symptoms to streptococcal tonsillitis.

Aplastic anemia (myelosuppression)

This condition is typical for otherwise very efficient antibiotic known as chloramphenicol. When using chloramphenicol, there frequently occurs a transient suppression of hematopoiesis in the bone marrow. The problem is that severe and persisting myelosuppression occurs in about one of 30,000 people treated with chloramphenicol. This complication may occur even many years after administration of the antibiotic. It is virtually incurable and fatal. That is the reason, why chloramphenicol is virtually never the antibiotic of choice (with exception of certain special rare indications)

Kidney damage

Many antibiotics are toxic to the kidneys (nephrotoxic) and it is necessary to pay attention in their administration to people with acute or chronic renal failure. Patients in these conditions should get only antibiotics that do not damage kidney and event these antibiotics have to be usually used in lower doses than normally. The most toxic antibiotics for kidney tissue are the aminoglycosides.

Hearing impairment

Some antibiotics from the group of aminoglycosides are toxic to the auditory nerve and may cause its permanent damage with lifelong deafness. This is the reason why these antibiotics should be used with extreme caution in patients with pre-existing hearing problems.


Despite the above side effects, antibiotics are great medications. However, as we know their disadvantages and possible risk, we should prescribe them wisely and cautiously. The choice of antibiotic and its dosage must be strictly individual depending on the patient's condition, type of infection and sensitivity of the present bacterium to antibiotics.  Only this is probably an effective measure to reduce both the side effects and the growing resistance of bacteria to antibiotics.


Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Sources: basic text sources