Amoxicillin vs. Penicillin: Allergy & Difference

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson goes over the differences and similarities between penicillin and amoxicillin. You'll also learn some very important allergy information that applies to both.


Antibiotics, drugs that kill or prohibit bacteria from multiplying, are very diverse. The most famous family of antibiotics are collectively called beta-lactams. Among other kinds of antibiotics, beta-lactams include the cephalosporins and penicillins. You read that right, penicillins (plural).

There is actually more than one kind of penicillin. The most famous penicillin is a basic (natural) penicillin called penicillin G, the first to be discovered, and penicillin V, one that is given by mouth. Penicillin G or penicillin V are the types of penicillin most people refer to when they simply say 'penicillin'. But other types of penicillins have been developed since those two. This includes the semi-synthetic aminopenicillins, like ampicillin and amoxicillin. This lesson goes over the differences between the basic (natural) penicillins and amoxicillin as well as important allergy information with respect to both.

For simplicity's sake, when this lesson refers to penicillin from now on, it refers to the basic (natural) penicillins even though amoxicillin is technically a type of penicillin as well.

Similarities and Differences


Let's start with some of the similarities. Both penicillin and amoxicillin:

  • Are bactericidal antibiotics. In other words they kill ('-cidal') bacteria.
  • Both are able to target gram positive bacteria, which are bacteria with thick cell walls.
  • Either one can target anaerobes, bacteria that dislike oxygen.
  • They are excreted out of our body mainly via our urine.
  • They are relatively ineffective at penetrating into specially protected areas of our body, like the brain and eyes, compared to other types of antibiotics (exceptions are possible, however, in some circumstances).
  • They have a relatively short half-life, which means they need to be given relatively more frequently than other types of antibiotics. Exceptions exist, however, depending on specifically how the two antibiotics are formulated.
  • Both are sensitive to beta-lactamases, enzymes produced by some kinds of bacteria that destroy these antibiotics and render them useless.


Now for some of the differences between penicillin and amoxicillin:

  • Penicillin's oral bioavailability is lowered by stomach acid more so than amoxicillin. In other words, if it's solely a choice between penicillin or amoxicillin when having to give the medication by mouth only, amoxicillin is the better choice.
  • Amoxicillin can target a broader range of bacteria, like more kinds of gram negative bacteria, bacteria with thin cell walls. This include E. coli, Haemophilus, and Klebsiella. While penicillin can target some gram negative bacteria, this usually requires higher concentrations of the drug, which may predispose to more adverse effects.
  • Amoxicillin is often combined with beta-lactamase inhibitors to improve its efficacy against bacteria that use beta-lactamase. Such inhibitors are called clavulanic acid, sulbactam and tazobactam. One famous medication that does this is called Clavamox, a band name for a combination of amoxicillin and clavulanic acid.

Allergy Information

People can be allergic to both penicillin and amoxicillin. A body's immune system can mistake the antibiotics for an invader and cause numerous problems, such as:

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