Difference Between Amoxicillin and Penicillin

Amoxicillin vs Penicillin

Antibiotics are produced by microbes including bacteria and actinomycetes usually in response to a stress or as secondary metabolites. These are effective against other species of bacteria and hence the term ‘antibiotics’. The discovery of antibiotics has led to increased use of the compounds as drugs. Amoxicillin and Penicillin are two such antibiotics.

Amoxicillin is an antibiotic belonging to the group of penicillin. Other members of this class include ampicilli, piperacillin etc. All of them have similar mechanism of action. They do not kill bacteria, but stop the microbes from multiplying. This is achieved by preventing the microbes to build cell walls around them. The bacteria require cell walls for protection and rigidity. Without cell wall they cannot survive and hence die off. The antibiotic forms differ in the spectrum of action or the microbes to which they are antagonistic. Amoxicillin is effective against many bacteria including H. influenzae, N. gonorrhoea, E. coli, Pneumococci, Streptococci, and certain strains of Staphylococci.

Penicillin is the first generation antibiotic having similar functions but differing in efficacy.


Amoxicillin is a semisynthetic aminopenicillin antibiotic structurally related to the penicillin family. There are similar structural analogs including ampicillin which provides similar function also. The moderate-spectrum antibiotic is effective against a wide range of Gram-positive bacteria and a limited number of Gram-negative microbes.

It is used to treat certain bacterial infections such as pneumonia; bronchitis; gonorrhea; and ENT infections, Urinary tract, and skin infections. Helicobacter pylori, bacteria that causes ulcers is susceptible to amoxicillin when used in combination with other drugs. The bacterial action is similar to that of penicillin by preventing the cell wall formation in bacteria.

The antibiotic has better absorption rates, and is the first choice of ear infection. It readily penetrates into the tissues and tissue fluids. The antibiotic cannot cross the brain and spinal fluid and hence not effective for brain tissues. It is effective and safe for use in high risk category of people including pregnant women and children.

The medicine is cheaper and safer as proved by more than forty years of research studies. Allergies are common and the drug is more likely to result in diarrhoea as side effect. It is not effective against bacterial species producing beta lactamase enzyme. A recent study has found a correlation between tooth enamel defects and increased use of amoxicillin infancy.


Penicillin is a narrow spectrum antibiotic effective against most Gram positive and a few Gram negative bacteria. The mode of action is similar with the inhibition of cell wall formation in the microbe. The antibiotic is effective against infections caused by streptococcus, staphylococcus, pneumococcus etc.

The prophylaxis is easier and treatment can be done by either oral or intravenous methods. The antibiotic is safer and may be taken with meals without getting inactivated by the gastric acid. The penetration levels are good in most tissues and come cheap. This is considered safe for pregnant women and children as proved from the studies. Being the first antibiotic to be discovered, it has undergone further modifications to suit the needs and enhance the efficacy.

The antibiotic has a very low half life requiring it to be administered once in six hours for optimal effect. The hypersensitivity associated with Penicillin has been historical and famous and is reported in numerous cases. The taste is not so appealing to children.

Difference between Amoxicillin and Penicillin

Absorption- Amoxicillin is better absorbed from the gastrointestinal tract compared to other Penicillins such as penicillin V and ampicillin. The levels of drug in blood are high and stable with administration of Amoxicillin.

Synthesis- Amoxicillin’s superior absorption may be attributed to the semi synthetic nature. Penicillin is synthetic penetrates less and hence less effective.

Efficacy- Amoxicillin is more effective and acts against a wide range of pathogenic microbes.

Penetration into tissues- Amoxicillin penetrates better into tissues than penicillin. The only exceptions are brain tissues and spinal fluid.

Safety- Both are suitable for use in pregnancy and in paediatrics.

Cost- Both antibiotics are cheaper and are available in generic formulations.

Duration of treatment- Treatment with Amoxicillin requires fewer courses of antibiotics compared to Penicillin. These can be taken for a short while.

Action- Both of them act on the bacteria by inhibiting cell wall formation.

Source- Both are isolated from molds.



1. Both belong to the class of Penicillin because of the similarity in action, structure and source of origin.

2. They differ in effectiveness in penetration which contributes to the efficacy.

3. Both have proven research confirming the safety in high risk category use.

4. They are cheaper generic versions and are available easily.

5. Hypersensitivity is a common issue for both drugs.