Ampicillin and amoxicillin are both aminopenicillins, with similar antibacterial spectra (effective against many Gram-positive organisms and some Gram-negative ones). However, there are some clinical and pharmacological differences that determine when ampicillin might be preferred over amoxicillin:

Advantages of Ampicillin :
A)Better for Certain Parenteral (IV/IM) Uses.Ampicillin is widely used intravenously (IV) or intramuscularly (IM) for serious infections (e.g., Listeria monocytogenes meningitis, enterococcal infections, or neonatal sepsis).
Amoxicillin is mostly used orally; IV formulations are less common.
Specific Indications (Especially in Hospital Settings)
B)Listeria infections: Ampicillin (often with gentamicin) is the drug of choice.
Enterococcus faecalis endocarditis: Ampicillin (often combined with gentamicin or ceftriaxone) remains standard due to synergistic effects.
C)Neonatal sepsis or meningitis: Ampicillin is preferred due to proven efficacy and safety in neonates.
D)Lower Oral Bioavailability (Sometimes an Advantage): Oral ampicillin has lower absorption (~40%) compared to amoxicillin (~70–90%).This can leave more drug in the gut, sometimes used intentionally for decolonization of susceptible bacteria in the intestines (though this is rare).
E)Cheaper in Injectable Forms:
In many regions, ampicillin IV is more affordable than IV amoxicillin or amoxicillin-clavulanate.

Amoxicillin Is Usually Preferred because of the following reasons .
1)Better oral bioavailability results more predictable blood levels.
2)Less frequent dosing (typically every 8–12 hours vs. ampicillin every 6 hours).
3)Better gastrointestinal tolerance (ampicillin often causes more diarrhea).
More palatable suspensions for children.

Ampicillin is generally preferred when IV administration is needed for serious infections.Also in Treating Listeria, Enterococcus, or neonatal sepsis.Cost-sensitive IV therapy is also a factor factor.
Otherwise Amoxicillin is better for most oral outpatient infections due to superior absorption, tolerance, and convenience.
Regards,
JAKSTAR PHARMA .