Ampicillin vs. Amoxicillin

Main Difference

The main difference between Ampicillin and Amoxicillin is that Ampicillin is slightly less lipid soluble whereas Amoxicillin is more lipid soluble.

Ampicillin vs. Amoxicillin — Is There a Difference?

Difference Between Ampicillin and Amoxicillin

Ampicillin vs. Amoxicillin

Ampicillin is slowly and partly absorbed from the gastrointestinal tract as compared to Amoxicillin.

Ampicillin vs. Amoxicillin

Ampicillin is more effective than Amoxicillin for shigellosis.

Ampicillin vs. Amoxicillin

Peak plasma concentration of Ampicillin is25 times less than those of Amoxicillin when taken orally at the same dose.

Ampicillin vs. Amoxicillin

The chances of diarrhea are more in Ampicillin as compared to Amoxicillin.


(pharmaceutical drug) A semisynthetic form of penicillin C16H19N3O4S that is effective against gram-negative and gram-positive bacteria and is used to treat infections of the urinary, respiratory, and intestinal tracts.


(pharmaceutical drug) A moderate-spectrum, bacteriolytic beta-lactam antibiotic C16H19N3O5S closely related to ampicillin and with similar properties, but more readily absorbed when taken orally. It is used to treat bacterial infections caused by susceptible microorganisms.



semisynthetic penicillin (trade names Principen and Polycillin and SK-Ampicillin)


an antibiotic; a semisynthetic oral penicillin (trade names Amoxil and Larotid and Polymox and Trimox and Augmentin) used to treat bacterial infections

Comparison Chart

Ampicillin is broad spectrum Aminopenicillin which is chemically an α-Aminobenzyl penicillinAmoxicillin is a broad spectrum and chemically a 6 - (p-Hydroxy-alpha-aminophenylacetamide) penicillanic acid

Susceptibility to β- lactamase

Ampicillin is destroyed by the β- lactamases(from gram positive and gram negative bacteria)Amoxicillin is destroyed by the β- lactamases(from gram positive and gram negative bacteria)

Gastrointestinal Absorption


Effect of Food on the Rate of Absorption

Intake of food before ingesting Ampicillin decreases its absorptionIntake of food before ingesting Amoxicillin does not affect its absorption

Available Dosage Forms

Capsules, Parenteral, Suspension, InfusionCapsules, Tablets, or Powder for Oral Suspension

Preferred Dosage Form

I/V, I/MOral

Therapeutic Uses

Urinary tract infections, Meningitis, Uncomplicated gonorrhea, Bacterial endocarditis, Prophylaxis of neonatal group B streptococcus infectionsTonsillitis, Bronchitis, Gonorrhea, and Ear, Nose, Throat, pneumonia skin or urinary infections.

Ampicillin vs. Amoxicillin

Ampicillin is penicillinase-susceptible penicillin and is the prototype drug while Amoxicillin is penicillinase-susceptible penicillin but is a synthetic analog of Ampicillin. Ampicillin inhibits cell wall synthesis in the third and final stage thus finally causes cell lysis. So Ampicillin is bacteriolytic but in the last stages of bacterial multiplication, on the other hand, Amoxicillin is more lipophilic thus its bactericidal action during active stages of multiplication is quicker as compared to Ampicillin. Ampicillin has smaller Vd in comparison with Amoxicillin while Amoxicillin has larger Vd in comparison with Ampicillin. The onset of action of Ampicillin through oral route is unknown while the onset of action of Amoxicillin through oral route is 30 minutes. Ampicillin causes side effects in penicillin-sensitive patients like allergies common to all penicillin-sensitive patients. On the other hand, Amoxicillin causes specific “Amoxicillin rash” in Amoxicillin sensitive patients. Ampicillin Overdose causes confusion, convulsions, blackouts, electrolyte imbalance while Amoxicillin Overdose causes seizures, bleedy diarrhea, and acute interstitial nephritis.

What is Ampicillin

Ampicillin is antibiotic classified as penicillinase-susceptible penicillin and is the prototype drug of this group. Its chemical name is α-Aminobenzyl penicillin. Penicillin the substituent at position 6 of the penam ring is a 2-amino-2-phenylacetamide group. Ampicillin is acid stable and absorbed poorly by gut due to the ionization of both amino and carboxylic groups. Presence of food also interferes with its absorption. Ampicillin can kill both Gram-positive and Gram-negative bacteria. The amino group of Amoxicillin helps it to pass through the outer membranes of Gram-negative bacteria. Amoxicillin is absorbed orally, and peak plasma concentration reaches in 1 to 2 hrs. Ampicillin is an irreversible inhibitor of the Transpeptidase, which is an enzyme needed by bacteria for the synthesis of the cell wall. Ampicillin is classified under the category of bacteriolytic. Ampicillin reacts with many drugs, e.g. when reacts with Probenecid and Methotrexate it decreases renal excretion. Large doses of ampicillin can increase the risk of bleeding when taken with warfarin and other oral anticoagulants. Oral contraceptives are less effective when taken with Ampicillin.

What is Amoxicillin

Amoxicillin is a semisynthetic broad-spectrum antibiotic classified as penicillinase-susceptible penicillin. It is effective against both Gram-positive and Gram-negative bacteria. Amoxicillin is stable in acid. .Amoxicillin has excellent absorption when taken orally. Absorption of Amoxicillin is not affected by food. The molecular formula of Amoxicillin is C16H19N3O5S•3H2O, and the molecular weight is 419.45. Due to its excellent absorption oral absorption, its concentration is high in blood and urine. Amoxicillin destroys cross-linkages of the peptidoglycan polymers from which the bacterial wall is made up of. Amoxicillin has two ionizable groups. Amoxicillin is mostly prescribed in the trihydrate form, but for the intravenous administration, Amoxicillin is prepared in sodium salt form. The extended-release dosage form is also available. The intravenous dosage form is preferred only when the patient can not take it orally. Amoxicillin has drug interactions like Amoxicillin makes birth control pills less effective. Allopurinol, probenecid, sulphonamides interact with Amoxicillin.


Both Ampicillin and Amoxicillin are Beta-lactamase sensitive penicillins. Both are preferred drug of choice depending upon the medical condition and have their beneficial effects.