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Understanding Antibiotics - How They Work, Side Effects, and Antibiotic Resistance

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Antibiotics fight bacterial infections. To know more about its uses, dosage, precautions, side effects, and interactions read below.

Medically reviewed by

Dr. Kaushal Bhavsar

Published At November 18, 2022
Reviewed AtDecember 22, 2023

What Are Antibiotics?

Antibiotics are common agents used to treat and prevent bacterial infections. However, doctors most frequently prescribe them to treat specific infections, as each antibiotic is effective only against certain types of bacteria. These antimicrobials are divided into two classes based on their mechanism of action:

Bacteriostatic: Antibiotics such as Clindamycin and Doxycycline kill the bacteria by interfering with their formation or cell content.

  • Glycylcyclines - Tigecycline.
  • Tetracyclines - Doxycycline, Minocycline.

  • Lincosamides - Clindamycin.

  • Macrolides - Azithromycin, Clarithromycin, Erythromycin.

  • Oxazolidinones - Linezolid.

  • Sulfonamides - Sulfamethoxazole.

Bactericidal: Antibiotics such as Gentamicin and Cephalosporins stop bacterial growth or reproduction.

  • Aminoglycosides - Tobramycin, Gentamicin, Amikacin.

  • Beta-lactams - Amoxicillin, Cefazolin, Meropenem.

  • Fluoroquinolones - Ciprofloxacin, Levofloxacin, Moxifloxacin.

  • Glycopeptides - Vancomycin.

  • Cyclic lipopeptides - Daptomycin.

  • Nitroimidazoles - Metronidazole.

How Are Antibiotics Metabolized?

On oral administration, the antibiotics move into circulation. However, absorption is affected by various factors, including stomach and intestine acidity, stomach food, and the antibiotic's chemical properties. As a result, they are distributed unevenly throughout the body, and the drug concentration in each organ varies in proportion to the antibiotic's blood plasma concentration. The dosage and the route of administration also determine the concentration in the blood plasma. In addition, the drug is inactivated in the liver and converted into readily excreted metabolites from the kidneys as urine.

What Do Antibiotics Treat?

Antibiotics are used to treat or prevent bacterial infections. They work by killing or preventing them from spreading. They are not used for viral infections such as colds, flu, or sore throat. Antimicrobials may only treat bacterial infections that are unlikely to clear without antibiotics or could take too long to clear without treatment. These include ear infections, bladder and kidney infections, whooping cough, bacterial pneumonia, or dental infections.

They treat bacterial infections that risk more severe complications, such as cellulitis, urinary tract infection, genital herpes, or rheumatic fever. Antibiotics are also used to treat those bacterial infections that can spread and infect others.

People with a high risk of infection may also be given antibiotics as a preventive dose before surgeries or chemotherapy treatment. In addition, antibiotics are recommended for people who are more vulnerable to the harmful effects of infections, such as people over 75 years, people with heart failure, or those who are immunocompromised.

How to Take Antibiotics?

Antibiotics can be most effective when used appropriately and should be taken only when prescribed. It is essential to take the antibiotics in the prescribed amount and for the directed duration of treatment, and they should not be stopped early, even if the symptoms subside. In addition, it should not be shared with others or given to someone else without a doctor's consent.

Antibiotics are available as:

  • Tablets, capsules, or a liquid to treat mild to moderate infections in the body.
  • Creams, lotions, sprays, and drops to treat skin, ear, or eye infections.
  • Injectables are given intravenously or intramuscularly for more severe infections.

What Are the Side Effects of Antibiotics?

The common side effects include,

  • Bloating.

  • Diarrhea.

  • Indigestion.

  • Feeling sick.

  • Nausea.

  • Vomiting.

  • Prolonged use may cause fungal infections of the mouth, digestive tract, and vagina.

  • Rash.

  • Abdominal pain.

  • Loss of appetite.

Serious side effects include,

  • Severe life-threatening allergic reactions.

  • Antibiotic resistance infections.

  • Bloody diarrhea.

  • Inflamed bowels.

  • Swelling of tongue and face.

What Is Antibiotic Resistance?

The overuse of antibiotics increases the chances of bacteria becoming resistant to them. It is a growing problem when antibiotics are taken unnecessarily for conditions such as colds and flu. Misuse of antibiotics also happens when a person has been prescribed the wrong antibiotic, dose, or length of time. An antibiotic-resistant infection will show no improvement after treatment with standard antibiotics. Young children, cancer patients, and people over 60 are more prone to antibiotic resistance.

Examples of antibiotic-resistant bacteria include

  • Clostridium difficile.

  • Vancomycin-resistant Enterococcus.

  • Methicillin-resistant Staphylococcus aureus.

  • Carbapenem-resistant Enterobacteriaceae.

What Are the General Precautions to Be Taken While Taking Antibiotics?

  • Liver Diseases - Antibiotics like macrolides such as Erythromycin, Azithromycin, Lincomycin, and Clindamycin excreted by the liver can increase the risk of toxicity in patients with liver disease.

  • Kidney Disease - Aminoglycoside antibiotics such as Neomycin and Gentamicin can cause kidney injury even in low doses in patients with chronic kidney disease.

  • Elderly - Inappropriate antibiotic therapy in the elderly can increase the risks of adverse consequences such as nerve damage.

  • Children - It is advised to avoid or limit antibiotics like fluoroquinolones and tetracyclines in children.

  • Alcohol - Drinking alcohol while taking antibiotics can lead to dehydration, an upset stomach, and a lower immune response. It is advised to avoid alcohol while taking antibiotics like Metronidazole, Tinidazole, Doxycycline, and Linezolid.

What Are the Warnings and Precautions to Consider While Taking Antibiotics During Pregnancy and Breastfeeding?

Some antibiotics like tetracyclines and sulfonamides can affect bone development and discolor the teeth of the developing baby or increase the risk of heart conditions, cleft lip, palate, and jaundice. Treatment with antibiotics like Bactrim and Macrobid during pregnancy can cause congenital disabilities, including brain malformations and heart defects. In addition, cleft lip and palate can pose a significant risk. Antibiotics passing through breast milk can disrupt an infant's gastrointestinal flora and cause diarrhea, rashes, or thrush.

The antibiotics are safely taken during pregnancy include,

  • Penicillin.

  • Ampicillin.

  • Amoxicillin.

  • Clindamycin.

  • Erythromycin.

What Are the Interactions of Antibiotics?

Drug Interaction:

  • Contraceptive Pills - Antibiotics like Rifampicin and Rifabutin can reduce the effectiveness of contraceptive pills.

  • Penicillin and Methotrexate - Combining these two medications can cause severe and unpleasant side effects.

  • Cephalosporins and Anticoagulants - Cephalosporins may increase the chance of bleeding if taken with blood-thinning medications such as Heparin and Warfarin.

  • Aminoglycosides with Antifungals, Cyclosporin, Diuretics, and Muscle Relaxants - Taking one or more of these medications can increase the risk of kidney and hearing damage.

  • Macrolides with Terfenadine, Statins, and Rivaroxaban - Combining these medications can cause heart problems and should be avoided unless directed by the physician.

  • Fluoroquinolones with Antacids, Zinc Supplements - It is advised to avoid taking high levels of minerals or iron formulations like antacids and multivitamins supplements with Fluoroquinolones as they can reduce their effectiveness.

Interaction With Food:

  • Dairy Products - Dairy products such as milk, yogurt, and cheese decrease the absorption of antibiotics like Tetracycline, Doxycycline, and Ciprofloxacin.

  • Grapefruit - Grapefruit juice should be avoided while taking antibiotics as it can interfere with the breakdown of antibiotics in the liver.

  • Fluoroquinolones and Carbonated Drinks - Fluoroquinolones can intensify caffeine's effects, make you irritable and restless, and cause insomnia.

  • Acidic Foods - Acidic foods like citrus juices, chocolates, antacids, and tomato-based products such as ketchup can interfere with drug absorption.

Disease Interaction:

  • Colitis - Antibacterial therapy alters the colon's normal flora, leading to the overgrowth of Clostridium difficile-associated diarrhea. Therapy with broad-spectrum antibacterials should be administered cautiously in patients with a history of colitis.

  • Mononucleosis - Therapy with Aminopenicillin antibiotics should be avoided in patients with mononucleosis.

  • Diabetes - High urine concentrations of Ampicillin may result in false-positive reactions when testing for the presence of glucose.

  • Renal Dysfunction - Prolonged therapy with beta-lactam antibiotics can cause nephrotoxicity and alter the renal function and should be monitored periodically with renal function tests.

  • Hemodialysis - Doses of Penicillin should be adjusted when given to patients on dialysis as the medication is removed by hemodialysis.

What Are the Common Brand Names of Antibiotics?

  1. Capsule Duricef 500 mg.

  2. Tablet Keflex 250 mg.

  3. Capsule Ceclor 500 mg.

  4. Capsule Omnicef 300 mg.

  5. Tablet Sivexyro 200 mg.

  6. Tablet Zithromax 500 mg.

  7. Tablet Biaxin 250 mg.

  8. Tablet Dificid 200 mg.

  9. Tablet Alinia 500 mg.

  10. Suspension Zyvox 100 mg / 5 ml.


Antibiotics should be taken responsibly and are most effective when used appropriately. This requires the effort of clinicians, practitioners, pharmacists, and nursing staff to use agents judiciously to improve therapeutic results with minimum adverse events.

Frequently Asked Questions


How to Choose Antibiotics for Use?

While selecting the antibiotic, the doctor may consider the following listed below.
- The drug's possible side effects.
- The nature and seriousness of the infection.
- Possibility of allergies or other serious reactions to the drug.
- Based on the patient's immune system.


What Should Be Avoided While on Antibiotics?

One should not take antibiotics:
- That are prescribed for other persons.
- Do not share antibiotics with others.
- Antibiotics should be taken with water.
- Avoid alcohol while on antibiotics.


Which Antibiotic Should Be Given First?

Amoxicillin or amoxicillin clavulanate is recommended as the first line of therapy.


How Much Water Should Be Consumed While Intaking Antibiotics?

Staying hydrated while taking antibiotics may help them work better.


What Is Basic Antibiotic Medicine?

Penicillin was a common treatment for a variety of skin infections. In addition, cephalosporins often treat gonorrhea, pelvic inflammatory disease, and sinusitis. Other antibiotics include glycopeptides, fluoroquinolones, tetracyclines, and sulfonamides.


What Is the Best Time of the Day to Consume Antibiotics?

Some antibiotics should be taken at a fixed time of the day and at regular intervals, and others should be taken before or after a meal. The physician will provide guidance and explain the modalities clearly in consultation.


How Long Do Antibiotics Stay In Our Bodies?

Each antibiotic may stay in the body for different lengths of time. Still, some common antibiotics, such as ciprofloxacin and amoxicillin, may stay in the body for around 24 hours after taking the last dose. It may take longer for people with impaired kidney function to eliminate the drug from the body.


Can One Take Antibiotics Before Going to Bed?

Generally, taking the antibiotic right before bedtime is not good because it can irritate the esophagus.


Can One Take Antibiotics Along With Milk?

It is best, as well as advised, to take antibiotics with water. The patients are advised against consuming antibiotics with dairy products and fruit juices. These products can interfere with the antibiotics and affect how the body absorbs them.


Is Paracetamol an Antibiotic?

No, paracetamol is not an antibiotic drug. It is an anti-inflammatory drug.


Will the Bacteria Go Away After Taking Antibiotics?

Each time one takes antibiotics, the bacteria present are killed. Unfortunately, in some cases, the bacteria causing the infection may resist the prescribed antibiotics.


What Are the Foods to Be Given With Antibiotics?

Foods that can be safely eaten with antibiotics include lightly salted crackers, peanut butter, and non-citrus fruit.


What Happens After Completing the Antibiotics?

If one stops the antibiotic before the cycle ends, the remaining bacteria may continue to multiply. These bacteria develop resistance to the antibiotics that can potentially harm them. As a result, it takes longer to recover from the illness, or the organisms may not respond to the antibiotics.
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Dr. Kaushal Bhavsar
Dr. Kaushal Bhavsar

Pulmonology (Asthma Doctors)


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