Penicillin refers to a group of antibiotics that kill or stop the growth of bacteria by weakening their cell walls. They’re among the oldest and most commonly used antibiotics. There are many different penicillins, with common examples including:
- Penicillin V (oral)
- Penicillin G (injected)
- Amoxicillin (oral)
- Ampicillin (oral or injectable)
- Penicillinase-resistant penicillins (e.g., dicloxacillin, flucloxacillin)
- Extended-spectrum penicillins (e.g., piperacillin, often given with tazobactam in hospital)
What they’re used for
- A wide range of infections caused by susceptible bacteria (e.g., throat infections like strep, some ear and sinus infections, skin infections, pneumonia, dental infections, and certain sexually transmitted infections like syphilis). The exact choice depends on the infection and the bacteria suspected or confirmed.
How they’re given
- Oral tablets or liquids, and some are given by injection or intravenously in a healthcare setting.
Common side effects and risks
- Usually mild: stomach upset, nausea, diarrhea, or skin rash.
- Allergic reactions can be serious (hives, swelling of the face/throat, trouble breathing). Severe reactions require urgent medical care.
- Less common issues include yeast infections or diarrhea from disruption of gut bacteria.
- Rare interactions with other medicines can occur; always tell your healthcare provider about other meds you take.
Important safety notes
- Do not take penicillin if you have had a serious allergic reaction to penicillin or a very similar drug in the past.
- There is some cross-reactivity with cephalosporin antibiotics, but the risk is relatively small. Your clinician will consider history of reactions.
- Finish the full course as prescribed to clear the infection and help prevent resistance.
- If you’re pregnant or breastfeeding, penicillins are often considered safe, but you should still use them only under a clinician’s guidance.
- If you’re on birth control, some antibiotics can affect effectiveness; use back-up contraception as advised by your clinician.
- If you have kidney disease, your dose or choice of penicillin may be adjusted.
If you want, tell me:
- Which penicillin (or infection) you’re asking about
- Your age and pregnancy status
- Any allergies or other medications you take
I can then give more specific general information (not a replacement for medical advice). If you have symptoms suggesting an infection or you’ve been prescribed penicillin and have concerns, contact a healthcare provider. And seek urgent care if you experience signs of a severe allergic reaction.