Amoxicillin

What it is

Amoxicillin is one of the most prescribed oral antibiotics in Spain and around the world. It belongs to the aminopenicillins, an evolution of penicillin G with better oral absorption and a broader spectrum.

In Spain it can only be supplied on a medical prescription. Using it without one is illegal and contributes to bacterial resistance, one of the most serious public health problems of the 21st century.

Bactericidal: it blocks the building of the bacterial cell wall

Amoxicillin acts on actively growing bacteria:

• It binds to penicillin-binding proteins (PBPs) in the bacterial membrane.
• It inhibits transpeptidase, a key enzyme in the synthesis of peptidoglycan (a component of the bacterial wall).
• The bacterium cannot maintain its wall → osmotic lysis and bacterial death.
• It is BACTERICIDAL (it kills bacteria), not just bacteriostatic (slowing their growth).
• Spectrum: Gram-positives (Streptococcus, Enterococcus) and some Gram-negatives (H. influenzae, E. coli, H. pylori).
• It does not act on bacteria without a wall (Mycoplasma, Chlamydia) or on beta-lactamase producers (without an inhibitor).

At the counter

When to recommend it

• Supply on a valid prescription: check that it is correctly prescribed
• Remind the patient to take it for as long as the doctor has said, even if symptoms improve in 2–3 days
• Can be taken with or without food (it does not affect absorption)
• Offer a probiotic (Lactobacillus or Saccharomyces boulardii): it reduces antibiotic-associated diarrhoea
• If there has been a previous reaction to penicillins, refer to the doctor before supplying

When not to

• Without a prescription: in Spain it is illegal and clinically dangerous
• Viral infections (flu, colds, COVID): it has no antiviral effect at all
• Known allergy to penicillins (risk of anaphylaxis): always check
• Infections by beta-lactamase-producing bacteria without an inhibitor (many S. aureus and E. coli): it does not work

Warnings

• Important PENICILLIN ALLERGY: it can cause potentially fatal anaphylaxis in sensitised patients. Always ask about allergies before supplying. There is partial cross-reactivity with cephalosporins.
• Important VIRUSES: amoxicillin has no effect at all on viruses. Giving it for flu or a cold not only does not help, it also contributes to selecting resistant bacteria.
• Caution INFECTIOUS MONONUCLEOSIS (Epstein–Barr virus): amoxicillin in patients with mononucleosis produces a very striking widespread skin rash. If the patient has mononucleosis, it should be avoided.
• Caution ORAL CONTRACEPTIVES: current guidance considers that amoxicillin does not reduce their efficacy. What can do so is vomiting or severe diarrhoea shortly after the pill: that is what needs explaining.
• Worth knowing RESISTANCE: overuse of amoxicillin has led to significant resistance in H. influenzae and E. coli. That is why co-amoxiclav (amoxicillin-clavulanic acid) is used when beta-lactamase-producing bacteria are suspected.