Penicillin and amoxicillin are antibiotics, compounds that disrupt and destroy bacteria. Penicillin is the precursor to amoxicillin, and both antibiotics are derived from a mold called Penicillium glaucum. The discovery of penicillin's effect on bacteria led to a revolution in medical treatment and the development of dozens of other antibiotics, including amoxicillin, which is a cheaper antibiotic that treats a wider range of gram-positive bacteria and is less likely to cause allergic reactions.

Amoxicillin was originally patented and sold under the trade name Amoxil. When the patent expired, many other patentable amoxicillin/clavulanic acid combinations of the drug were developed, including the well-known Augmentin, which is also no longer under patent. Derivatives of amoxicillin are extremely common and found under numerous names.

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Amoxicillin versus Penicillin comparison chart
Edit this comparison chartAmoxicillinPenicillin
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What It Is Antibiotic based on penicillin that attacks cell wall structure of bacteria. Does not work on viruses. Antibiotic based on Penicillium glaucum mold that attacks cell wall structure of bacteria. Does not work on viruses.
Chemical Origin Penam structure of penicillin Penicillium glaucum
Treats Wider range of gram-positive bacteria than penicillin. A limited number of gram-positive bacteria.
Can Cause Allergic Reaction? Yes, but less likely than penicillin to do so. Yes
Common Side Effects Nausea, vomiting or stomach pain; headaches; white patches inside the throat or mouth (thrush); swollen, black or "hairy" tongue; yeast infection. Nausea, vomiting or stomach pain; headaches; white patches inside the throat or mouth (thrush); swollen, black or "hairy" tongue; yeast infection.
Serious Side Effects Watery/bloody diarrhea; easy bruising/bleeding; yellowing of eyes/skin; frequent coughing or trouble breathing; severe rashes; flu-like symptoms; changes in behavior; severe tingling, numbness or weakness; little or no urination; seizures/convulsions Watery/bloody diarrhea; easy bruising/bleeding; yellowing of eyes/skin; frequent coughing or trouble breathing; severe rashes; flu-like symptoms; changes in behavior; severe tingling, numbness or weakness; little or no urination; seizures/convulsions
Drug Interactions Interferes with birth control pills; methotrexate (Rheumatrex, Trexall); probenecid (Benemid); because of allergy risk, all other medications being taken (supplements, herbal remedies, etc.) should be noted before treatment. Interferes with birth control pills; methotrexate (Rheumatrex, Trexall); probenecid (Benemid); because of allergy risk, all other medications being taken (supplements, herbal remedies, etc.) should be noted before treatment.
Safe During Pregnancy? Yes, Category B Yes, Category B
Safe During Breastfeeding? No No
Safe for Children? Yes, over the age of 5, under medical supervision. Yes, over the age of 10, under medical supervision.
Cost Tablets (30, 500 mg each): $4.00-$12.79 Tablets (40, 500 mg each): $10.00-$37.20

How Penicillin and Amoxicillin Work

The cell walls of bacteria are constantly broken down and rebuilt as part of their rapid growth cycle. Penicillins disrupt this cycle by penetrating deep into a bacterium's developing cell wall to prevent the wall from solidifying and growing stronger. This weakens and eventually kills bacterial cells. For an example of penicillin's effects on E. coli bacteria, see this video.

Bacteria that lose their cell walls during mitosis (cell division) are called gram-positive; those that don't lose their cell walls completely are called gram-negative. Penicillins are much more effective against gram-positive bacteria.

Forms and Dosage

Penicillin is used in three ways: in IV solution as Penicillin G, orally as Penicllin V, and in intramuscular (IM) injections, such as as procaine benzylpenicillin or benzathine benzylpenicillin. Amoxicillin is almost always used in oral form because it is best absorbed through the gastrointestinal tract. It is usually prescribed to children more than traditional penicillin because amoxicillin is easier to take (no needles involved) and because children are more prone to ear and throat infections, conditions amoxicillin treats quite well.

Dosages for both penicillin and amoxicillin vary according to a patient's weight, age, and condition, with lower doses prescribed to people who have not used a penicillin before (to determine allergy risk). In general, when the allergy risk is minimal or nonexistent, dosages begin towards the middle range of the appropriate age/weight/condition spectrum and are adjusted upward if no strong positive result (reduced level of infection) is noted within 8-10 hours in the case of a monitored hospital stay.

Blood tests or bacterial swabs are taken to verify the level of bacteria present in an infection. If required, a course of penicillin, amoxicillin, and/or other antibiotics is prescribed for treatment, usually for a period of 5-10 days, with 3-4 pills taken a day (in the case of oral forms). A round of antibiotics must be taken, as prescribed, and in its entirety, even if symptoms disappear after a few days of use.


Penicillins are used to treat bacterial infections of all kinds. The first successful penicillin treatment was for eye infections, in adults and infants. Skin infections were also responsive to the antibiotics, and by the time World War II broke out, penicillin became a common treatment for battlefield wounds and sexually-transmitted diseases, with varying results. It was during the 1940s and 1950s that researchers discovered penicillin is ineffective against viral infections. Viruses are basically DNA strands that lack a cell structure and so are not affected by an antibiotic's cell wall attacks.

Penicillin is most effective against strep and soft tissue infections (mainly caused by Staphylococcus strains), syphilis, meningitis, and pneumonia. Amoxicillin is effective against most of the same strains as penicillin, but is more effective against otitis media (ear) infections, endocarditis (heart valve infections), and infections caused by enteroccocus strains.


Natural penicillins and synthesized versions, such as amoxicillin, are frequent weapons in the medical arsenal against disease due to their effectiveness. Not only can they cure bacterial infections, they can also prevent subsequent bacterial infections from occurring. This has led many doctors, veterinarians, and the agricultural industry to overprescribe the use of antibiotics, which in turn has led to the evolution of antibiotic-resistant bacteria.

Amoxicillin and penicillin are often equally effective in treating a wide variety of infections, from medical to dental. As such, amoxicillin is often prescribed simply because it is cheaper. However, one antibiotic may be prescribed for a certain type of infection more than another. For example, amoxicillin was found to decrease swelling caused by abscessed primary ("baby") teeth better than penicillin, making amoxicillin the preferred antibiotic for this type of infection.[1]

Antibiotic Resistance

One of the most virulent antibiotic-resistant bacterial strains in humans is Methiciliin-Resistant Staphyloccus aurea, commonly referred to by its acronym, MRSA (often pronounced mur-suh). While Staphyloccus aurea was once a form of bacteria easily killed by penicillins, its multi-resistant form is now a "flesh-eating disease" that is capable of destroying tissue in hours and resisting a wide variety of heavy antibiotic treatments.

Despite resistant strains, antibiotics are still effective in controlling and defeating most bacterial infections. Awareness of overusing antibiotics has curtailed their use somewhat in favor of alternative treatments, or, as in the case of colds and flus, which are mostly caused by viruses, letting the illness proceed untreated unless a bacterial infection develops.

There is some evidence that prescribed doses of penicillin can be lowered, yet remain highly effective. Amoxicillin, more than penicillin, appears to remain effective at a lower dose. If antibiotic doses can be lowered, the potential for the development of "superbugs" may decrease. However, even with antibiotic-resistance being a concern, patients should defer to their doctors' recommendations, as dosing requirements are often closely related to the type of infection.

Penicillin Side Effects

Penicillin can cause an allergic reaction in about 10% of the population. However, the allergic reaction can fade over time if the person is not exposed again, with only about 20% of the allergic remaining so some 10 years after their initial exposure. An allergic reaction to any one of the penicillins is enough to presume one is allergic to all of them.

In some cases, the allergic reaction can be quite severe, resulting in shock that can be fatal. Those who have had any prior allergic reaction to penicillin, amoxicillin, or related antibiotic formulations, should tell their doctors before taking any similar type of medication. People with asthma, bleeding, or clotting disorders, kidney diseases or a history of diarrhea should tell their doctors about the condition(s).

Because penicillin and amoxicillin are primarily excreted renally (through urine), people with kidney ailments or renal conditions must be careful when taking these types of antibiotics.

Common Side Effects

Common side effects of penicillin and amoxicillin include:

Amoxicillin shows a lower rate of side effects than penicillin, but dosages still must be followed with care as per medical instructions. Other side effects may occur with either medication and should be mentioned to a doctor.

Serious Side Effects

The serious side effects of penicillin and amoxicillin often include:

Amoxicillin has proven to elicit fewer serious side effects than penicillin, especially in children. However, any of the serious side effects requires prompt medical attention.

Women who are pregnant can take penicillin or amoxicillin, under medical supervision. However, women who are breastfeeding should not use either medicine as it can pass through to the baby and cause serious side effects.

Reduction of "Good" Bacteria

As penicillins do not distinguish between "good" and "bad" bacteria, intestinal flora can be seriously affected during treatment and for weeks afterward. This bacterial reduction is what leads to diarrhea, yeast infections, flu-like symptoms, and/or reduced water and nutrient absorption (decreased urination as the body tried to retain water). To offset these side effects, some doctors and pharmacists recommend taking a probiotic while on antibiotics.

Drug Interactions

Penicillin and amoxicillin interfere with oral contraceptives ("the pill" birth control), rendering them less effective. If using birth control pills and antibiotics, a woman can become pregnant, so other forms of birth control are needed.

Anyone taking methotrexate (Rheumatrex, Trexall) or probenecid (Benemid), should tell his or her doctor about these and other medications. Penicillin and amoxicillin can enhance or inhibit the effects of these and other medications, especially those related to gastrointestinal and kidney functions. Patients should also tell their doctors about any vitamins, supplements, and/or herbal remedies they are currently using to avoid serious or even fatal drug interactions.


Amoxicillin is considerably cheaper than penicillin, but neither antibiotic is very expensive. According to, Penicillin V Potassium pills (40 tablets of 500 mg each) range in price from $10.00 to $37.20. Amoxil, a brand name for amoxicillin (30 tablets of 500 mg each) ranges from $4.00 to $12.79.

History of Penicillin

Ernest Duchesne, a French physician, first noticed the microbial-inhibiting effect of the Penicillium mold in 1897. Despite using the mold to cure typhoid in guinea pigs, Duchesne's paper on the experiment was ignored. Penicillin, as such, was identified and isolated by Scottish physician Alexander Fleming, in 1928, using Penicillium rubens. Fleming isolated the mold's substance and proved it was non-toxic in humans, but the development of penicillin as a medicine was completed by Howard Florey, Ernst Chain, and Norman Heatley, an Austrian-German-British collaboration for which Florey and Chain won the Nobel Prize.

Because penicillin was difficult to produce and badly needed during World War II, treatments were limited to severe cases of infection. Efforts to make the best use of penicillin often included collecting the urine of treated patients to "recycle" the medicine, as about 80% of penicillin is excreted within 3-5 hours. This proved ineffective and efforts to increase the time penicillin remained in the body led to the discovery of pairing it with probenecid, which blocked the body's natural "flushing" of penicillin and allowed the medicine to work for a longer period of time.

Once biosynthesis of penicillin became common and large amounts of the medicine were readily available, probenecid was eliminated from most treatments, although it is still used for particularly aggressive bacterial infections and in cases where resistant bacterial strains, such as MRSA, are present, or for treating H. pylori, the bacteria that causes most stomach ulcers.

In 1961, ampicillin became the first penicillin-based antibiotic developed in a laboratory that used the penam structure. The semi-synthetic formulation quickly proved to be as effective as other penicillins against most bacterial infections, but with the added advantage of resulting in fewer side effects. Within a year of its development, it was in widespread use and opened the door to new formulations of penicillins, including amoxicillin, which entered the market in 1972.


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