Understanding how long antibiotics take to work, and what they are used for, is vital for patients. The NHS provides clear guidance to help people use these medications responsibly and effectively.
-
When do antibiotics start working?
Antibiotics start acting against bacteria from the moment you take them, but feeling better can take longer. According to NHS information, you should start to feel better within a few days of taking most antibiotics. For example:
- With clarithromycin (a macrolide), many people feel improvement within a few days. For skin infections like cellulitis, it may take up to 7 days to notice significant results.
- Metronidazole may begin working in a few days. For certain infections (e.g., dental abscesses), it still may take a bit longer before symptoms improve. In topical forms (e.g., for rosacea), improvement can take a few weeks.
- Amoxicillin: If you don’t feel better within 3 to 5 days, or if symptoms worsen, you’re advised to contact your doctor.
- Erythromycin (another macrolide): Expect improvement within a few days; skin conditions like acne may take at least 2 months to improve.
- Ciprofloxacin (a fluoroquinolone): Most infections respond in a few days, but tougher infections like bone infections (osteomyelitis) may take a week or longer.
Generally, most antibiotic courses last between 5 and 14 days, depending on the infection type and severity.
-
Commonly used antibiotics and their uses
Per NHS guidance, the main antibiotic groups include:
- Penicillin (e.g., penicillin, amoxicillin, co-amoxiclav, flucloxacillin, phenoxymethylpenicillin): Widely used for skin, chest, and urinary tract infections.
- Cephalosporins (e.g., cefalexin): Treat a wide range of infections, including serious ones like sepsis and meningitis.
- Aminoglycosides (e.g., gentamicin, tobramycin): Typically reserved for serious hospital-based infections due to side-effect risks.
- Tetracyclines (e.g., doxycycline): Commonly used for acne and rosacea.
- Macrolides (e.g., clarithromycin, erythromycin, azithromycin): Used for lung/chest infections or for those allergic to penicillin.
- Fluoroquinolones (e.g., ciprofloxacin): Broad-spectrum, once common for respiratory and urinary infections but now limited due to serious side effects.
Other frequently prescribed antibiotics include trimethoprim and nitrofurantoin for UTIs. Studies show that amoxicillin, flucloxacillin, and trimethoprim rank among the most commonly prescribed antibiotics in UK primary care.
-
What antibiotics should not be used for
The NHS emphasises that antibiotics are ineffective against viral infections, including the common cold, flu, most coughs, sore throats, and many ear infections.
Overuse or misuse, such as prescribing antibiotics for viral illnesses, contributes to the serious problem of antimicrobial resistance (AMR).
Other examples of inappropriate use include:
- Using oral antibiotics for conditions better treated with topical treatments (e.g., swimmer’s ear).
- Treating sinusitis, which is usually viral, unnecessarily with antibiotics.
-
If you finish your course but symptoms persist
NHS guidance is clear:
- Always complete the full course, even if you feel better earlier – that helps prevent recurrence and resistance.
- If you’ve finished your course and don’t feel better, or feel worse, see your doctor as soon as possible.
- Certain antibiotics may require follow-up – for example, with clarithromycin for H. pylori, a follow-up test is usually arranged 6–8 weeks after starting treatment.
- For sepsis, hospital protocols call for antibiotics within 1 to 6 hours of arrival due to the urgency of treatment.
Speak to your GP
If you have been prescribed antibiotics by your GP, ensure you take them exactly as instructed by your doctor, and complete the full course. If your symptoms persist after completing your course of antibiotics, be sure to speak to your GP again.
