The connection between ampicillin and amoxicillin is that the drugs are both antibiotics and members of the class known as penicillins. The precise action of the drugs is slightly different, but they both work by weakening the cell walls that surround bacteria, rendering them vulnerable to attack by white blood cells. Both ampicillin and amoxicillin can be used to treat infections of the inner ear and gonorrhea. They are both proven to be effective against E. coli, streptococci, and H. influenzae bacteria, amongst others. Both drugs also cause similar common side effects in patients.

Ampicillin and amoxicillin are both members of the penicillin class of antibiotics. Antibiotic drugs are used to help the body fight different strains of bacteria which cause infection and illness. Ampicillin and amoxicillin, as penecillins, are similar in the way they fight bacteria.

Bacteria need a protective cell wall to survive within the human body — the wall acts as a shield to protect them from the attacking white blood cells. Antibiotics such as ampicillin and amoxicillin break down this wall, which makes it easier for white blood cells to kill the bacteria. Amoxicillin breaks down the wall by disrupting its polymer chains, while ampicillin removes a basic component needed to build the wall. Metaphorically, amoxicillin messes around with the bricks in the wall so it falls down and ampicillin breaks all the bricks before they are made into a wall.

Different strains of bacteria require different antibiotics to treat them. This is because bacteria are not all susceptible to the same tricks employed by the drugs to break down the cell wall. Both ampicillin and amoxicillin are suitable for use against E. coli, H. influenzae, streptococci, N. gonorrhea, and some strains of staphylococci. Ampicillin is also effective against salmonella, however, unlike amoxicillin.

The similar action of the drugs means that they can affect similar illnesses. For example, both treatments can be used against gonorrhea and middle ear infections. Ampicillin is also used against more serious infections such as meningitis and endocarditis. Amoxicillin also works against pneumonia and bronchitis.

Side effects produced by ampicillin and amoxicillin are similar. Common side effects of both drugs vomiting, abdominal pain, and dizziness. Diarrhea and nausea are two other common side effects which can be caused by both treatments. Patients may also suffer allergic reactions with both of the treatments. The similarity between the actions of ampicillin and amoxicillin mean that many patients will be allergic to both treatments.