Ampicillin: What it Treats and How to Use It Safely

Ampicillin is a common antibiotic from the penicillin family. Doctors use it for infections caused by certain bacteria — for example ear infections, bronchitis, urinary tract infections, and some intestinal infections. It comes as capsules, oral suspension, or injection, so the form you get depends on how serious the infection is.

How ampicillin is usually taken

Dosage varies by infection, age, and kidney function, so follow your prescriber. As a rough example, adults often take 250–500 mg by mouth every 6 hours for mild infections. For more serious cases given intravenously, doses like 1–2 g every 4–6 hours are common in hospital settings. Kids get a dose based on weight. These are examples only — never change dose or stop treatment without checking with your clinician.

Take the oral form with a full glass of water. If you use the liquid suspension, shake it well and measure with a proper dosing spoon or syringe — kitchen teaspoons aren’t reliable. Keep a regular schedule so levels stay steady in your body. If you miss a dose, take it as soon as you remember unless it’s almost time for the next one; don’t double up.

Side effects, allergies and interactions

Common side effects are stomach upset, diarrhea, and sometimes a mild skin rash. Many people tolerate ampicillin well, but watch for signs of a serious allergic reaction: hives, swelling of face or throat, trouble breathing. If that happens, get emergency help right away. If you’ve had a severe reaction to penicillin before, tell your doctor — ampicillin is usually avoided.

Ampicillin can interact with other drugs. Tell your prescriber about blood thinners, methotrexate, certain diuretics, and any other antibiotics you’re taking. Also mention supplements and herbal products. If you’re on immunosuppressants or have kidney problems, the dose may need adjustment.

Diarrhea while on antibiotics can be just a side effect, but if it’s severe, bloody, or persistent, contact your doctor — that could be a sign of Clostridioides difficile infection, which needs quick treatment.

Practical tips: finish the full course even if you feel better early, because stopping too soon can let bacteria survive and become resistant. Don’t share your medicine or use leftover antibiotics for a new illness. Store tablets and capsules in a cool, dry place; keep liquid suspensions refrigerated if the label says so and throw out any leftover after the prescribed time.

If you’re pregnant, breastfeeding, or have chronic health issues, discuss risks and benefits with your healthcare provider. When used correctly, ampicillin is a useful antibiotic, but it works best when paired with good communication with your doctor and careful use.