Antibiotics: side effects to know


Antibiotics are prescribed to fight against certain bacterial infections. But what precautions should one take when using these drugs? Fatigue, risk of yeast infection, bowel disorders … What are their side effects and how to limit them? Answers from Magalie Le Bihan, pharmacist.

Definition and mode of action: what is an antibiotic?

When you are sick, a doctor may prescribe an antibiotic. And for good reason, the antibiotic is a drug capable of killing an infection caused by a bacterium by preventing it from developing or reproducing. There are different types of antibiotics: it will be up to the doctor to decide which one is best suited to fight the infection the patient has. The healthcare professional knows when to prescribe an antibiotic and when it is not. Indeed, ” when a doctor prescribes an antibiotic, it is because he has evaluated upstream the benefits and the risks for the patient. An antibiotic prescribed wisely must always have a real benefit for the patient “, confirms of 

Note that some antibiotics, such as penicillin, the first antibiotic discovered in 1929 by Fleming, are historically natural, but most are now synthetic antibiotics.

Indications: when to take an antibiotic?

“Antibiotics are precious, let’s use them as best as possible”, urges the Ministry of Health in its new slogan unveiled at the end of 2018. Note that an antibiotic should only be prescribed to treat infectious diseases of bacterial origin such as:

Conversely, the antibiotic cannot kill a virus and is therefore ineffective in treating viral diseases such as influenza , nasopharyngitis, acute bronchitis, or most tonsillitis. Likewise, the antibiotic does not help relieve symptoms of a viral illness such as fever, cough, headache, or digestive upset. To alleviate the symptoms, medications are available. Ask your pharmacist for more information. 

Instructions for using them properly

When a healthcare professional prescribes an antibiotic for you, it is necessary to:

  • well meet the dose, frequency and timing of the catch, and the duration of the prescribed treatment (even if your condition improves);
  • do not exchange antibiotics with another person: each treatment is specifically administered according to the type of infection to be treated and the patient’s profile (age, weight, medical history, etc.);
  • tell your doctor about any side effects of the treatment;
  • do not reuse an antibiotic for the months following the end of treatment, even if you present a priori the same symptoms as before;
  • return any opened or unused boxes to your pharmacy at the end of the treatment. 

“Before taking an antibiotic, you must be sure that you have never had an allergy to this molecule or therapeutic family in the past. It is therefore important to note your allergies, for example in your health record or to ask his attending physician or at his pharmacy to record them in their software “, advises Magalie Le Bihan. In addition, some antibiotics can be photosensitizing and cause allergic reactions if you expose yourself to the sun. However, ” it is the role of the doctor and the pharmacist to warn the patient before the administration of an antibiotic”, she adds. 

Examples of antibiotics

The main families of antibiotics are:

  • beta-lactams  (including penicillins , the best known of which are amoxicillin, augmentin and cephalosporins ): Amodex®, Clamoxyl®, Amoxicil®, Augmentin®, Alfatil®, Cefixim® …
  • aminoglycosides: Nebcine®, Tobi®
  • macrolides: Azadose®, Clarithromycin® …
  • (fluoro) quinolones: Ciflox®, Levofloxacin®, Ofloxacin® … 
  • les cyclines : Doxy®, Spanor®, Doxylis®, Tolexine®, Doxycycline®, Lymécycline®…

Side effects of antibiotics

Bowel disorders

All antibiotics can cause intestinal disorders of various types, depending on the molecule they contain and the susceptibility of each patient. ” Indeed, antibiotics act against the” bad “bacteria responsible for infection, but also the” good “bacteria that colonize the intestinal microbiota. The balance of the intestinal microbiota being modified, digestive symptoms such as nausea , vomiting , bloating, or even intestinal spasms may appear, “confirms the pharmacist. 


“This is not the most reported side effect of antibiotics, even if it is mentioned in the leaflet of certain drugs “, underlines the specialist. However, how do you tell the difference between fatigue from infection and fatigue from treatment ?” If it is not scientifically proven that taking an antibiotic causes fatigue, the loss of intestinal flora could explain the feeling of being more weakened, since the patient will expend more energy during digestion by example, ”she says. 

Yeast infection

“The most common skin condition after taking an antibiotic is allergy-related hives . However, the antibiotic may be responsible for yeast infections of the mouth or vagina, since this drug destroys certain bacteria in the skin. oral or vaginal flora “, explains the specialist. These yeast infections are generally caused by yeasts that can develop in the absence of good bacteria whose role is to prevent their proliferation. 


Antibiotics can upset the intestinal flora,  which normally ensures good digestion. “Indeed, antibiotics have the effect of neutralizing the bacteria responsible for the infection, but also attack certain” good “bacteria in our intestines. This can in particular cause diarrhea. This is the effect. undesirable majority of antibiotics”, explains the expert. To prevent diarrhea , it is possible to take probiotics which will better regulate your digestive system and repopulate the intestine with “good” bacteria. 

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Precautions: limit side effects

To limit the side effects and increase the effectiveness of the treatment, it is important to respect the timing indicated on the prescription and always ask your pharmacist for details in case of doubt. “Depending on the molecules they contain, some antibiotics are taken during, before or at a distance from meals , while for others, the time of taking does not matter,” says Magalie Le Bihan. 

In addition, it is better to take probiotics if you have already had symptoms, including diarrhea or yeast infection after taking antibiotics. The best is to seek advice from your pharmacist who is well trained on these products. He may direct you to probiotics specially designed to offset the effects of antibiotic treatments, or to probiotics that act on the prevention of yeast infections … ” It takes 3 to 6 months for the intestinal flora to reconstitute itself. So if we continue (especially children!) antibiotic dragging, it may be worth taking probiotics in prevention, “she recommends. 


“There are no more undesirable effects in people” at risk, “but contraindications or dosage adjustments to be observed “, specifies the specialist. However, certain classes of antibiotics are totally prohibited in:

  • the child,
  • the pregnant woman,
  • in the event of hepatic insufficiency , in this case, “we will prefer the drugs not metabolized by the liver”, she specifies,
  • in renal failure because “many antibiotics and drugs in general are eliminated through the kidneys, so there is a risk of accumulation of the molecule or its metabolites”
  • in heart failure. 

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