Antibiotic Classes and Adverse Effects
On this page
Direct answer
Each antibiotic class works by a distinct mechanism, and that mechanism largely predicts both its spectrum and its characteristic toxicity. Beta-lactams inhibit cell-wall synthesis, aminoglycosides and tetracyclines act on the 30S ribosomal subunit, and fluoroquinolones inhibit DNA gyrase and topoisomerase IV. Matching each class to its signature adverse effect is one of the most reliable scoring habits in NEET-PG pharmacology.
What you must remember
- Beta-lactams: Penicillins and cephalosporins commonly cause hypersensitivity reactions, and high-dose imipenem can provoke seizures, particularly with renal impairment.
- Aminoglycosides: Nephrotoxicity and ototoxicity dominate, and neuromuscular blockade can cause respiratory paralysis in susceptible patients.
- Fluoroquinolones: Tendon rupture, mainly of the Achilles tendon, QT prolongation and dysglycaemia are the key toxicities, and cartilage damage limits use in children.
- Macrolides: Erythromycin and clarithromycin prolong the QT interval, erythromycin estolate causes cholestatic jaundice, and clarithromycin inhibits CYP3A4.
- Tetracyclines: Photosensitivity, oesophagitis and intracranial hypertension are typical, and tooth staining with enamel hypoplasia rules them out in young children and pregnancy.
- Linezolid: Myelosuppression, especially thrombocytopenia with prolonged use, serotonin syndrome with SSRIs, and optic or peripheral neuropathy.
A worked choice: empirical therapy in an older adult with pneumonia
A 68-year-old with community-acquired pneumonia and COPD needs empirical therapy. A respiratory fluoroquinolone looks attractive for its coverage, but the team first checks tendinopathy risk, since age over 60 and concurrent corticosteroids both raise it. They then ask about other QT-prolonging drugs such as ondansetron, and an ECG is reviewed with potassium corrected. A macrolide is chosen if the QT risk is low. In a child with an atypical pneumonia, tetracyclines are generally avoided because of tooth discolouration and enamel damage, so a macrolide is used instead. Doses are adjusted for renal function, and the patient is told to stop a fluoroquinolone and seek help at once if tendon pain appears, because delay can end in rupture. Once the pneumonia responds, the team reviews whether an oral switch or a shorter course is appropriate, and counts the QT-prolonging drugs still on the chart before discharge. Discharge counselling covers tendon warning signs and the need to report palpitations or fainting, and the same review applies to any child or pregnant patient before the next dose. The final chart is checked against allergy and interaction records, which is how the classic errors are caught in practice.
Where the exam tries to catch you
The commonest error is attaching a toxicity to the wrong class. Macrolides and fluoroquinolones both prolong the QT interval, but only fluoroquinolones cause tendon rupture and cartilage damage, so a stem describing tendon pain should lead to a fluoroquinolone rather than a macrolide. Metronidazole is taught to cause a disulfiram-like reaction with alcohol, a classic viva point, although some modern reviews question how strong the evidence is. Tetracycline tooth staining is often mistaken for an allergy, yet it is a direct chelation effect on developing teeth. Linezolid's serotonin risk with SSRIs is an interaction question rather than a toxicity of linezolid alone.
Frequently asked questions
Which antibiotic classes prolong the QT interval?
Macrolides, especially erythromycin and clarithromycin, and fluoroquinolones, particularly moxifloxacin, can prolong the QT interval. Risk rises with hypokalaemia, structural heart disease and co-prescription of other QT-prolonging drugs such as ondansetron, so an ECG is advisable in high-risk patients.
Which antibiotic class is most associated with tendon rupture in adults?
Fluoroquinolones are most associated with tendon rupture, with the Achilles tendon the commonest site. Risk is highest in patients over 60, those taking corticosteroids and transplant recipients, and the drug should be stopped at the first sign of tendon pain.
Why are tetracyclines avoided in children under eight years?
Tetracyclines bind calcium in developing teeth and bones, causing permanent yellow-grey discolouration and enamel hypoplasia. This is why classic teaching avoids them in children under eight years and in pregnancy, unless no safer alternative exists.
Which antibiotic causes a disulfiram-like reaction with alcohol?
Metronidazole causes flushing, palpitations and vomiting when alcohol is taken during therapy and for a period afterwards, and tinidazole shares this effect. Classic teaching attributes the reaction to inhibition of aldehyde dehydrogenase, so patients are advised to avoid alcohol while on the drug.