Ampicillin dosage, forms, and strengths

Ampicillin is a generic type of penicillin antibiotic. Brand names for this antibiotic include Ampi, Omnipen, Penglobe, and Principen. This drug is approved by the United States Food and Drug Administration (FDA) for the management and prevention of several different bacterial infections usually found in the ear, bladder, lungs, gastrointestinal tract, or for treatment of sexually transmitted infections. Ampicillin is most commonly taken by mouth as a capsule but liquid forms can also be administered. 

Ampicillin forms and strengths

The dosage forms available for ampicillin include oral capsules or liquid suspensions. There is also a powder that can be made into an injection form. The options are as followed:

• Capsules: 250 milligrams (mg) or 500 mg

• Oral suspension: 125 mg or 250 mg per 5 milliliters (mL)

• Powder: 125 mg, 250 mg, 500 mg, 1 gram (g), 2 g, or 10 g*for reconstitution in injection form

Ampicillin dosage for adults

Ampicillin is commonly used for a variety of respiratory tract infections, genitourinary tract infections, gastrointestinal tract infections, meningitis, endocarditis, and the prevention of bacterial infections. Here is a list of different bacteria and conditions that ampicillin can effectively treat:

Types of bacteria

• Streptococcus pneumoniae

• Staphylococcus aureus

• Group A beta-hemolytic Streptococci

• Haemophilus influenzae

• Neisseria meningitidis

• Escherichia coli (E. coli)

• Listeria monocytogenes

• Group B Streptococci

• Enterococcus species

• Salmonella species

• Proteus mirabilis

• Shigella species

• Bacillus anthracis 

Types of medical conditions

• Bacterial meningitis

• Septicemia

• Endocarditis

• Urinary tract infections

• Typhoid infection

• Skin infections

• Pharyngitis

• Bronchitis

• Gonorrhea

• Surgery prophylaxis

• Newborn infection prophylaxis

* There are off-label treatments for other health conditions that are not listed.

Ampicillin dosage for bacterial infections (skin and soft tissue)

Ampicillin is approved for the use of treating broad-spectrum bacterial infectious diseases.

• Take 250-500 mg capsules by mouth every 4-6 hours.

• Give 1-2 g IV every 4-6 hours.

• Do not exceed 14 g per day.

Ampicillin dosage for meningitis

Ampicillin is approved for the treatment of bacterial meningitis but is usually due to E. coli, group B streptococci, and other gram-negative bacteria (Neisseria meningitidis). Combining an aminoglycoside may increase efficacy.

• Give 1-2 g IV every 3-4 hours for 7-21 days depending on the type of bacteria and severity of the infection.

Ampicillin dosage for GBS prophylaxis

Ampicillin is approved for perinatal Group B Streptococcal disease to prolong premature rupture of membranes.

• Give 2 g IV once at least 4 hours prior to delivery and then 1g IV every 4 hours until delivery.

• Maximum treatment is for 48 hours.

Ampicillin dosage for endocarditis

Ampicillin is approved for the treatment of endocarditis. Enterococcal strains work best with IV preparations and parenteral preparations are effective against susceptible gram-positive organisms. Many times an aminoglycoside antibiotic will be given in combination with ampicillin to increase its efficacy.

• Give 2 g IV every 4 hours for 4-6 weeks.

Ampicillin dosage for endocarditis prophylaxis

Ampicillin is approved for the prevention of endocarditis for various surgical procedures.

• Give 2 g IV/IM once 30-60 minutes prior to procedure.

Ampicillin dosage for typhoid fever

Ampicillin has been approved for the treatment of typhoid fever due to Shigella species, Salmonella species, E. coli, Proteus mirabilis, and enterococci. 

• Give 500 mg orally, IV, or IM every 6 hours.

Ampicillin dosage for gastrointestinal infections

Ampicillin has been approved for a variety of gastrointestinal infections due to Shigella species, Salmonella species, E. coli, Proteus mirabilis, and Enterococci. 

• Give 500 mg orally, IV, or IM every 6 hours.

Ampicillin dosage for respiratory tract infections

Ampicillin has been approved for a variety of respiratory tract infections due to S. pneumoniae, Staphylococcus aureus (penicillinase and non-penicillinase producing), H influenzae, and group A beta-hemolytic streptococci. Examples of conditions treated include pneumonia, bronchitis, and sinusitis.

• Give 250-500 mg orally, IV, or IM every 6 hours.

Ampicillin dosage for septicemia

Ampicillin is approved for the treatment of sepsis for gram-positive organisms including Streptococcus species, penicillin G-susceptible staphylococci, enterococci, and gram-negative sepsis due to E. coli, Proteus mirabilis, and Salmonella species.

• Give 1-2 g IV every 3-4 hours for at least 3 days and then can be switched to IM injections.

Ampicillin dosage for genitourinary tract infections

Ampicillin has been approved for a variety of urinary tract infections due to Shigella species, Salmonella species, E. coli, Proteus mirabilis, and enterococci. Also used for the treatment of uncomplicated gonorrhea sexually transmitted infections.

• Give 500 mg orally, IV, or IM every 6 hours.

• For gonorrhea, give 3.5 g oral dosage once in addition to probenecid 1 g. 

Ampicillin dosage for children

Ampicillin may be used in neonates, infants, and children for many of the conditions described above, including bacterial infections, severe bacterial pneumonia, meningitis, endocarditis, and typhoid fever.

* Pediatric dosages should not exceed recommended adult dosages.

Ampicillin dosage for pediatric bacterial infections (skin and soft tissue)

• Neonates younger than 7 days old:

  • Give 50-100 mg/kg IM/IV every 12 hours (weight less than 2 kg).

  • Give 75-150 mg/kg IM/IV every 8 hours (weight greater than 2 kg).

• Neonates 7 days old and older:

  • Give 50-100 mg/kg IM/IV every 12 hours (weight less than 1.2 kg).

  • Give 75-150 mg/kg IM/IV every 8 hours (weight between 1.2-2 kg).

  • Give 100-200 mg/kg IM/IV every 6 hours (weight greater than 2 kg).

• Infants/children:

  • Give 100-400 mg/kg IM/IV every 4-6 hours.

  • Maximum dosage: 12 g IM/IV daily.

Ampicillin dosage for pediatric community-acquired pneumonia, severe

• Pediatric patients at least 3 months old:

  • Give 150-400 mg/kg IV every 6 hours for 10 days.

Ampicillin dosage for pediatric meningitis 

Group B Streptococcal

• Neonates younger than 7 days old

  • Give 200-300 mg/kg IM/IV every 8 hours for 2-3 weeks.

• Neonates 7 days old and older

  • Give 300 mg/kg IM/IV every 6 hours for 2-3 weeks.

Non-Group B Streptococcal

• Neonates less than 7 days old:

  • Give 100 mg/kg IM/IV every 12 hours (weight less than 2 kg).

  • Give 150 mg/kg IM/IV every 8 hours (weight greater than 2 kg).

• Neonates 7 days old and older:

  • Give 100 mg/kg IM/IV every 12 hours (weight less than 1.2 kg).

  • Give 150 mg/kg IM/IV every 8 hours (weight between 1.2-2 kg).

  • Give 200 mg/kg IM/IV every 6 hours (weight greater than 2 kg).

• Infants/children:

  • Give 200-400 mg/kg IM/IV every 4-6 hours.

  • Maximum dosage: 12 g IM/IV daily.

Ampicillin dosage for pediatric endocarditis 

• Give 200-300 mg/kg IV every 4-6 hours for 4-6 weeks.

• Maximum dosage: 12 g IM/IV daily.

Ampicillin dosage for pediatric endocarditis prophylaxis

• Give one 50 mg/kg IM/IV dose 30-60 minutes prior to procedure.

• Maximum dosage: 2 g IM/IV dose.

Ampicillin dosage for pediatric typhoid fever

• Give 100 mg/kg IV every 6 hours for 1-2 weeks.

Ampicillin dosage restrictions

People considering ampicillin for the management of a bacterial infection should avoid this medication if there is a history of an allergic reaction to other penicillin medications such as amoxicillin. 

Also, ampicillin is not recommended for the treatment of mononucleosis due to the concern of developing a severe skin rash. 

Patients with renal impairment that have a creatinine clearance between 10 to 50 mL/min should divide the dose of ampicillin between every six to 12 hours. Those with a creatinine clearance of less than 10 mL/min must take the dose every 12 to 24 hours. Patients receiving hemodialysis should consider taking a supplement if their next dosage is not right after their dialysis treatment. 

There are no known hepatic dosage modifications needed for those with liver impairment.

Providing a thorough medical history to your healthcare provider is important when ampicillin is being prescribed due to potential side effects. If you have a prior history of any of the following, please review this with your provider:

• Diabetes

• Diarrhea from antibiotic use

• Previous reaction to cephalosporin antibiotics

• Seasonal allergies (hay fever)

• Asthma 

• Kidney problems

• HIV infection

• History of seizures

• Allopurinol usage

Ampicillin dosage for pets

Ampicillin is also available for off-label use in dogs, cats, and small animals, as most medications used by veterinarians usually only have approval for human use through the FDA. This medication should only be prescribed by a veterinarian, as it is based on the weight of the animal being treated. It is usually prescribed as a capsule, tablet, oral suspension, or occasionally as an injection given by the veterinarian. 

• Standard oral dosage in dogs and cats: 5-10 mg per pound 3-4 times a day for as long as the veterinarian feels it is needed.

How to take ampicillin

The most common way ampicillin is usually taken is by mouth. This may include tablets, capsules, or oral suspensions. Occasionally, your healthcare provider may also decide to inject ampicillin into your body. Here are some helpful tips to use when asked to take ampicillin:

• Your doctor will tell you how much to take and how often.

• Shake the oral liquid before each use.

• It is important that you keep taking the medicine for as long as your doctor tells you, even if you feel better.

• Take the medicine on an empty stomach, one hour before or two hours after meals with a full glass of water.

• Measure your medicine with a marked measuring spoon or medicine cup.

• Be sure to keep medication at room temperature away from excessive heat and moisture. 

Ampicillin dosage FAQs

What happens if I miss a dose of ampicillin?

It is important to take any prescription medication as directed from the drug information provided. If a dose is missed, take it as soon as possible unless it is close to taking the next scheduled dose. Do not take two doses together to make up for the missed dose. 

How long can you take ampicillin?

The length of time needed to take ampicillin will depend on the type of infection being treated and the severity of the infection. Your physician will let you know how long to take the medication. It is important that you complete your antibiotic and not discontinue the medication too soon even if your symptoms are improving.

Can you overdose on ampicillin?

Taking more medication than intended or prescribed may have the potential of causing life-threatening complications. There may be serious reactions due to the effects of ampicillin that need to be reviewed. They include hives, difficulty breathing, and swelling of the face, lips, tongue, or throat. If any of these things occur, seek medical advice from your physician or call the Poison Help line at 1-800-222-1222.

Other common or less severe reactions from the use of ampicillin that should be monitored closely include: 

• Nausea

• Vomiting

• Diarrhea

• Urticaria

• Rash

• Black hairy tongue

• Yeast infection

What interacts with ampicillin?

It is important that certain medications are avoided so that severe adverse effects from the use of ampicillin do not occur. If there has been a previous hypersensitivity to other penicillins or anaphylactic response to cephalosporin antibiotics, ampicillin should not be used. All prescription drugs, over-the-counter herbal supplements, and even vaccines should be reviewed with your provider. Drug interactions are also common with medications such as allopurinol. The following drugs should be avoided when using ampicillin:

• Amifampridine

• BCG live intravesical

• Bremelanotide

• Live typhoid vaccine

Other drug interactions may occur.

What happens when you mix ampicillin and alcohol?

Currently, there are no known contraindications for use of ampicillin and the consumption of alcohol. However, the combination should always be discussed with your healthcare provider.

Is it safe to take ampicillin during pregnancy?

Ampicillin is considered safe to use for pregnant women. Ampicillin can pass through the mother’s breast milk so breastfeeding while using this antibiotic does pose a potential risk of the infant developing diarrhea or thrush. Ampicillin may also make birth control less effective, so an alternative birth control plan may be needed temporarily. 

Related resources for ampicillin dosages:

• Ampicillin, StatPearls 

• Adult dosing, Epocrates

• Pediatric dosing, Epocrates

• Ampicillin, VCA

• Ampicillin (Polyflex), PetPlace

• Drug interactions, Epocrates

• Pregnancy/lactation, Epocrates