When prescribing even the most common antibiotics, doctors are advised to err on the side of caution by weighing the benefits against the potential risks. A recent study published in the Canadian Medical Association Journal (CMAJ) has revealed that amoxicillin and amoxicillin-clavulanic acid can often lead to unexpected side effects, such as diarrhea and candidiasis — a fungal infection in the mouth or throat, also known as thrush.

Amoxicillin is a commonly prescribed oral tablet used to treat bacterial infections in various parts of the body. Amoxicillin and other drugs belonging to the group of medication known as penicillin antibiotics work by killing bacteria and preventing their growth. Although amoxicillin is generally not prescribed to treat cold, flu, or other common virus infection, it is often used in combination with clavulanic acid to treat respiratory infections like bronchitis.

"The root cause of antibiotic resistance is the overuse of antibiotics, and therefore these drugs should not be prescribed when the benefits do not outweigh the harms," lead researcher Dr. Christopher Del Mar said in a statement. "The important consequence of under-reporting of harms is the tilting of the balance of benefits and harms toward amoxicillin."

Del Mar and his colleagues analyzed 45 clinical trials, including 27 that involved amoxicillin, 17 that involved amoxicillin-clavulanic acid, and one that involved both antibiotics. A total of 10,519 children and adults were included in the systematic review, 4,280 of which received amoxicillin, 1,005 amoxicillin-calvulanic acid, and 5,234 received a placebo. Diarrhea was twice as common among people who took amoxicillin and three time more common among people who took amoxicillin-clavulanic acid compared to those who received a placebo. Thrush was also linked to both amoxicillin and amoxicillin-calvulanic acid.

"Reported harms were fewer than we expected from clinical anecdotal experience and observationally-derived data, which have primarily reported common harms as rashes (at rates of five percent to eight percent of those treated and even higher, up to 20 percent, among those with mononucleosis treated with amoxicillin) and gastrointestinal disturbance,” the authors added. "Under-reporting of harms in trials remains widespread, and until that problem is addressed, under-reporting will flow to systematic reviews and other evidence syntheses such as guidelines."

According to the Centers for Disease Control and Prevention, antimicrobial agents, such as amoxicillin, have been used to treat infectious disease as far back as the 1940s. Due to how much and how long these drugs have been used to treat infection, infectious organisms have adapted to them resulting in antibiotic resistance. Around two million people in the United States are infected with bacteria that are resistant to antibiotics each year, including 23,000 people who die as a result of these infections.

Source: Gillies M, Ranakusuma A, Hoffmann T, Thorning S, McGuire T, Glasziou P, Del Mar C. Common harms from amoxicillin: a systematic review and meta-analysis of randomized placebo-controlled trials for any indication. Canadian Medical Association Journal (CMAJ). 2014