Antibiotics — Medication Guide, Uses & Safety — Uses, Dosage & Side Effects | MyMedicPlus

Antibiotics are a class of prescription medications that kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria. They represent one of the most transformative advances in modern medicine, saving hundreds of millions of lives from previously fatal bacterial infections since penicillin was introduced in the 1940s. Antibiotics span multiple chemical classes — including penicillins (amoxicillin), cephalosporins (cefalexin), macrolides (azithromycin, clarithromycin), fluoroquinolones (ciprofloxacin, levofloxacin), tetracyclines (doxycycline), aminoglycosides (gentamicin), carbapenems (meropenem), and glycopeptides (vancomycin). Each class has a different spectrum of activity, targeting specific types of bacteria. Antibiotics are only effective against bacterial infections and have absolutely no effect on viral illnesses such as the common cold, most sore throats, influenza, or COVID-19. Misuse of antibiotics for viral infections is a major driver of global antibiotic resistance — a public health crisis that threatens the effectiveness of these life-saving drugs. Always use antibiotics only when prescribed by a qualified healthcare professional.

Antibiotics treat a wide range of bacterial infections across virtually every organ system. Common on-label uses include urinary tract infections (UTIs — amoxicillin, trimethoprim, nitrofurantoin), community-acquired pneumonia (amoxicillin, doxycycline, azithromycin), acute bacterial sinusitis, otitis media (middle ear infection), skin and soft tissue infections (cellulitis — flucloxacillin, cephalexin), sexually transmitted infections (chlamydia — doxycycline; syphilis — benzylpenicillin; gonorrhea — ceftriaxone), Helicobacter pylori eradication (combination triple therapy), Lyme disease (doxycycline), bacterial meningitis (ceftriaxone, penicillin G), and tuberculosis (combination rifampicin, isoniazid, pyrazinamide, ethambutol). Antibiotics are also used as surgical and dental prophylaxis to prevent infection in high-risk patients. Immunocompromised patients (HIV, chemotherapy, organ transplant recipients) require antibiotics for infections that a healthy immune system could otherwise handle. Choice of antibiotic is ideally guided by culture and sensitivity testing to identify the specific organism and its susceptibility pattern.

Different antibiotic classes target different essential bacterial processes, each exploiting structural differences between bacteria and human cells. Beta-lactams (penicillins such as amoxicillin, and cephalosporins such as cephalexin) block penicillin-binding proteins (PBPs) involved in bacterial cell wall synthesis — weakening the cell wall until the bacterium bursts due to osmotic pressure. Macrolides (azithromycin, clarithromycin) and tetracyclines (doxycycline) inhibit bacterial ribosomes (30S or 50S subunits), blocking protein synthesis. Fluoroquinolones (ciprofloxacin, levofloxacin) block DNA gyrase and topoisomerase IV, preventing bacterial DNA replication. Aminoglycosides (gentamicin) cause misreading of mRNA during protein synthesis. Metronidazole damages bacterial DNA through production of toxic free radicals. The right antibiotic is selected based on the suspected or confirmed type of bacterium, its sensitivity profile, the infection site, patient allergies, age, and organ function.

Antibiotic dosing depends on the specific antibiotic, the type and severity of infection, the patient's age, weight, and kidney and liver function. Examples of common dosing regimens: amoxicillin 500 mg three times daily for 5–7 days (respiratory infections); doxycycline 100 mg twice daily for 7 days (chlamydia, Lyme disease, respiratory infections); azithromycin 500 mg once daily for 3 days (atypical pneumonia); ciprofloxacin 500 mg twice daily for 3–7 days (UTI, GI infection); trimethoprim 200 mg twice daily for 3–7 days (UTI). Course lengths range from 3 days for uncomplicated UTIs to 6–12 months for tuberculosis. Always complete the full prescribed course of antibiotics even if symptoms improve before the course ends — stopping early may allow surviving bacteria to develop resistance and re-infect. Never share antibiotics between individuals or reuse leftover antibiotics from a previous prescription. Follow your doctor's instructions precisely.

Common side effects across most antibiotic classes include nausea (10–15% of users), diarrhea (5–20%), abdominal cramps, and oral or vaginal yeast infections (thrush — caused by disruption of the normal microbiome, particularly with broad-spectrum antibiotics). These gastrointestinal effects are usually mild and transient, and can be minimized by taking antibiotics with food where appropriate. Serious adverse effects include severe allergic reactions — ranging from skin rash and urticaria to anaphylaxis (potentially fatal; incidence up to 0.05% with penicillins). Seek emergency help immediately for any signs of anaphylaxis (swelling of face/throat, difficulty breathing, collapse). Fluoroquinolones (ciprofloxacin, levofloxacin) carry a risk of tendon rupture (Achilles tendon most commonly), peripheral neuropathy, and serious CNS effects. Tetracyclines (doxycycline) cause photosensitivity — use sun protection. Aminoglycosides are nephrotoxic (kidney-damaging) and ototoxic (ear-damaging) and require blood level monitoring. Prolonged or broad-spectrum antibiotic use can cause Clostridioides difficile colitis — presenting as severe, watery diarrhea — which may require specific treatment. Always disclose all antibiotic allergies to your prescriber before starting treatment.

Antibiotic resistance is a rapidly escalating global health crisis — inappropriate antibiotic use selects for resistant bacteria that cannot be treated with standard antibiotics, making common infections potentially fatal. Never take antibiotics for viral infections such as colds, flu, or most sore throats. Never take antibiotics without a proper medical prescription. Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) carry FDA black-box warnings for tendon rupture (risk increased in patients over 60 or on corticosteroids), peripheral neuropathy that may be permanent, and aortic aneurysm — reserve for infections with no safe alternatives. Penicillin allergy is one of the most commonly reported drug allergies (up to 10% of patients report it, though only 1% are truly allergic) — always inform your prescriber so an appropriate alternative can be selected. Key drug interactions: many antibiotics significantly increase warfarin (INR increases, raising bleeding risk); rifampicin dramatically reduces the effectiveness of oral contraceptives, antiretrovirals, and many other drugs; metronidazole causes severe nausea and vomiting with alcohol (disulfiram-like reaction — avoid alcohol entirely during treatment and for 48 hours after). Antacids, calcium, iron, and dairy products reduce fluoroquinolone and tetracycline absorption — take these antibiotics 2 hours apart from these products. Probiotics (Lactobacillus species) taken during and for 2 weeks after antibiotic courses can reduce the risk and severity of antibiotic-associated diarrhea.