Key takeaways
Ampicillin is available as capsules and injections.
The standard ampicillin dosage for bacterial infections in adults and children who weigh at least 88 lbs is 250 to 500 mg taken every six hours.
The maximum dosage of ampicillin is 2 g per day (administered orally) or 12 g per day (administered by injection).
The best time to take ampicillin is on an empty stomach, 30 minutes before or two hours after a meal.
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How much ampicillin should I take? Quick reference |
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|---|---|---|---|
| Patient | Typical starting dose | Standard dosage | Maximum dose |
| Adults | 250–500 mg every 6 hours | 250–500 mg every 6 hours | 2 g/day |
| Children (< 88 lbs.) | 50–100 mg/kg/day divided into 4 daily doses | 50–100 mg/kg/day divided into 4 daily doses | 100 mg/kg/day |
The dosages listed are general guidelines; your healthcare provider may adjust your dose based on individual health factors.
Ampicillin is a generic penicillin antibiotic known by various brand names, including Omnipen and Principen. It works as a bactericidal antibiotic that kills bacteria by destroying their cell walls and is FDA-approved for treating various bacterial infections in adults and pediatric patients. Most people typically take ampicillin capsules four times a day, but healthcare professionals may administer injections intramuscularly (IM) or intravenously (IV), depending on the type of infection.
Detailed ampicillin dosage chart for adults |
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|---|---|---|
| Indication | Standard dose | Maximum dose |
| Gastrointestinal tract or urinary tract infections (UTIs), except for gonorrhea | 500 mg every 6 hours | 2 g per day |
| Respiratory tract infections or soft tissue infections | 250 mg every 6 hours (capsule) OR 250–500 mg every 6 hours (IM/IV) | 2 g per day (capsule) OR 12 g daily (injection) |
| Gonorrhea | 3.5 g single dose alongside 1 g of probenecid OR 500 mg IM/IV injection every 6 hours (in women) or 500 mg once, then again after 8–12 hours (in men) | Determined by a healthcare provider |
| Bacterial meningitis | 150–200 mg/kg IV given in equally divided doses every 3–4 hours | 12 g per day |
| Severe infections, including septicemia | 150–200 mg/kg IV given in equally divided doses every 3–4 hours | 12 g per day |
Ampicillin dosage by weight in children |
|
|---|---|
| Weight | Recommended dosage (capsule) |
| Less than 20 kg | 50–100 mg/kg/day in 4 divided doses |
Ampicillin dosage by age in infants (for bacterial meningitis or septicemia) |
|
|---|---|
| Age | Recommended dosage (injection) |
| 7 days or less | 100 mg/kg IM/IV per day in 2 divided doses (every 12 hours) |
| 8 to less than 28 days | 150 mg/kg IM/IV per day in 2 divided doses (every 12 hours) |
| 28 days or older | 150 mg/kg IM/IV per day in 3 divided doses (every 8 hours) |
Personalized dosing considerations
People with kidney problems
may need to take the medication less often, with potential changes in dosage amounts.
For mild renal impairment (creatinine clearance of over 50 mL/min): Doses are taken every six hours (four doses daily)
For moderate to severe renal impairment (creatinine clearance of 10 to 50 mL/min): Doses are taken every six to 12 hours (two to four doses daily)
For severe renal impairment (creatinine clearance less than 10 mL/min): Doses are taken every 12 to 24 hours (one to two doses daily)
For patients on hemodialysis: A possible supplemental dose may be required after dialysis
No dosage modifications are necessary for people with liver problems or hepatic impairment.
Ampicillin safety info
Ampicillin is considered a safe antibacterial medicine for most people to take. Side effects are minimal. The most serious problems associated with ampicillin are severe skin reactions, liver problems, Clostridioides difficile-associated diarrhea, and hypersensitivity reactions, such as anaphylaxis.This medication should be avoided in individuals with a history of allergic reactions to penicillin drugs, such as amoxicillin or penicillin V, or cephalosporins, including cephalexin or ceftriaxone. It should also be avoided in people who have experienced liver problems from ampicillin in the past.
Ampicillin is not recommended for the treatment of infectious mononucleosis due to the risk of developing a severe skin rash.
Clostridioides difficile-associated diarrhea is caused by a bacterial infection of the large intestine. In addition, life-threatening intestinal swelling (pseudomembranous colitis) can occur in some individuals. These problems may arise months after ampicillin treatment has stopped. Immediately contact a healthcare professional if you experience severe diarrhea in the days, weeks, or months following ampicillin therapy.
To prevent possible adverse effects, provide the prescriber with a thorough medical history before starting ampicillin. Tell a healthcare provider if you have a prior history of:
Diabetes
Diarrhea from antibiotic use
Previous reaction to penicillin or cephalosporin antibiotics
Asthma or any other kind of allergy
Kidney or liver problems
To avoid possible drug interactions, review all prescription drugs, over-the-counter medications, supplements, and vaccines with a prescriber. Some drugs may need to be avoided when taking ampicillin, including:
Zyloprim (allopurinol): This drug may increase the chance of a skin rash when used with ampicillin.
Benemid (probenecid): This medication may slow the kidney removal of ampicillin, which can increase ampicillin levels.
Certain bacteriostatic antibiotics: Antibiotics such as chloramphenicol, erythromycin, sulfonamides, or tetracyclines may reduce the effectiveness of penicillins against bacteria; your prescriber will determine if they should be used in combination.
Hormonal birth control: Some reports have shown possible breakthrough bleeding and reduced effectiveness with ampicillin. Evidence is limited, and most people do not experience an interaction, but backup contraception may be recommended if vomiting or diarrhea occurs.
Ampicillin is considered safe for use during pregnancy. Ampicillin may pass through the mother’s breast milk while breastfeeding. However, the levels are so low that they aren’t expected to cause adverse effects in nursing infants.
How to take ampicillin
Ampicillin is usually taken by mouth as capsules. In severe cases, an injection of ampicillin can be given into the muscle or vein. Other dosage forms, such as a liquid version (oral suspension), can be prepared by a compounding pharmacy for individuals who are unable to swallow capsules.
Follow all the healthcare provider’s instructions.
Take each dose for the full duration of treatment.
Space out doses as evenly as possible throughout the day based on the prescribed dosage.
Take doses on an empty stomach at least 30 minutes before or two hours after eating.
Swallow the capsule whole with a glass of water.
Store at room temperature.
Ampicillin dosage FAQs
What happens if I miss a dose of ampicillin?
If a dose is missed, take it as soon as possible. If the next scheduled dose is near, skip the missed dose and take the next dose as scheduled. Do not take two doses at the same time.
How long can you take ampicillin?
Ampicillin is typically used for short-term treatment, usually lasting a few days to a few weeks. The treatment duration will vary depending on the type and severity of the infection.
Can you overdose on ampicillin?
Yes. An ampicillin overdose may cause serious digestive system problems and neurological problems, including seizures. The most common signs of an overdose are digestive system complaints such as nausea and diarrhea. If too much ampicillin is taken, stop taking the medicine and call either a poison control line or a healthcare provider.
What happens when you mix ampicillin and alcohol?
Ampicillin is not contraindicated with alcohol, but drinking while taking antibiotics might worsen side effects like nausea and diarrhea. Always check with a doctor about alcohol consumption when being treated for an infection.
What is the dosage of ampicillin for dogs/cats?
Veterinarians use ampicillin off-label to treat bacterial infections in pets. Dosages are based on the animal’s weight and are generally given as injections. The dosage will also vary depending on the infection being treated and the type of ampicillin being given.
Sources
Ampicillin capsule prescribing information
, DailyMed (2025)Ampicillin sodium injection prescribing information
, DailyMed (2025)Ampicillin
, StatPearls (2023)Ampicillin
, Drugs and Lactation Database (2025)Dosages of penicillins
, MSD Manual Veterinary Manual (n.d.)
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