Penicillin is the original antibiotic, and it is still in daily use. It treats streptococcal throat infections, syphilis, rheumatic fever prevention, dental and skin infections, and more. It comes as tablets and syrups for everyday use, and as long-acting injections for specific conditions.
What is Penicillin?
“Penicillin” is both a specific medicine and the name of a whole family. The South African Medicine Price Registry lists several distinct forms, all Schedule 4:
| Type | Products | Form |
|---|---|---|
| Phenoxymethylpenicillin (Penicillin V) | Incil VK, Betapen, Athlone Phenoxymethylpenicillin | Tablets 125/250 mg, suspensions |
| Benzylpenicillin (Penicillin G) | Bio-Pen 1MU, Bio-Pen 5MU | Injection |
| Benzathine benzylpenicillin | Penilente LA, Ultracillin | Long-acting injection |
| Procaine penicillin | Procillin | Injection |
The distinction matters more than the shared name suggests:
Penicillin V is acid-stable, so it survives the stomach and can be swallowed. It is the everyday oral form.
Penicillin G is destroyed by stomach acid and must be injected. It reaches much higher levels and is used for serious infections in hospital.
Benzathine penicillin is a depot injection — a suspension that releases slowly from the muscle over two to four weeks. That long action is the entire point of it, and it underpins two important uses.
All work the same way: they block bacteria from building their cell walls, so the organism cannot hold its shape and breaks apart. Penicillin kills bacteria rather than merely slowing them, and because human cells have no cell wall, it leaves them alone. That selectivity is why penicillin — discovered in 1928 — remains one of the safest and most useful antibiotics ever made.
What is Penicillin commonly used for?
Penicillin V (oral):
- Streptococcal throat infection (“strep throat”) and tonsillitis. This remains the first-choice treatment.
- Dental infections and abscesses, alongside dental treatment.
- Skin infections such as cellulitis and erysipelas.
- Preventing infection in people without a working spleen, or with certain heart conditions.
- Preventing recurrence of rheumatic fever, taken daily long-term.
Penicillin injections:
- Syphilis. A single benzathine penicillin injection treats early syphilis, and it remains the only recommended treatment in pregnancy. There is no oral substitute of equal standing.
- Rheumatic fever prevention, as a benzathine injection every three to four weeks for years. This is a major public health use in South Africa — untreated strep throat can cause rheumatic fever, which damages heart valves, and these injections prevent recurrence and further damage. The whole point of them is not missing one.
- Serious infections including meningitis, endocarditis and severe pneumonia, using high-dose benzylpenicillin in hospital.
- Diphtheria, tetanus and anthrax, as part of treatment.
What it does not treat: colds, flu, most sore throats and coughs — all viral. Most sore throats are viral, which is why a swab or a clinical scoring system is used before treating one as strep.
Finish the course. With strep throat this matters more than usual: a full ten-day course is what prevents rheumatic fever, and stopping early because the throat feels better is precisely how that protection is lost.
How Penicillin fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Natural penicillins | Penicillin V, penicillin G, benzathine penicillin | Strep throat, syphilis, rheumatic fever prevention |
| Aminopenicillins | Amoxicillin, ampicillin | Broader cover: chest, ear, urine, dental |
| Penicillin + inhibitor | Co-amoxiclav | Where resistance defeats plain amoxicillin |
| Penicillinase-resistant | Cloxacillin, flucloxacillin | Staphylococcal skin and soft tissue infection |
| Macrolides | Azithromycin (Azrotrin), erythromycin | The main alternative in penicillin allergy |
| Nitroimidazoles | Metronidazole | Anaerobic and dental infections |
Penicillin is narrow-spectrum, and that is a virtue rather than a limitation — a narrow antibiotic used for the right organism spares your normal bacteria and puts less pressure on resistance. See the antibiotics and antimicrobials hub.
What to check before using Penicillin
Penicillin allergy is the critical check, and it deserves more than a yes or no.
If you have ever reacted to penicillin, amoxicillin, cloxacillin, co-amoxiclav or a cephalosporin, say so before the first dose. Reactions range from a rash to anaphylaxis, and a previous mild reaction does not guarantee the next will be mild.
But also: many people labelled penicillin-allergic are not. A childhood rash during an illness is frequently recorded as an allergy and carried for life, and it matters — people labelled allergic get broader, less effective and more expensive antibiotics, with worse outcomes. If your allergy label is vague or dates from childhood, ask about being formally assessed. It is a genuinely useful thing to sort out.
Also tell your doctor or pharmacist if you:
- Have kidney disease — doses may need adjusting.
- Have glandular fever (infectious mononucleosis), where a rash is very likely.
- Take warfarin, methotrexate or probenecid.
- Use the combined oral contraceptive pill and develop vomiting or diarrhoea during the course.
- Are pregnant or breastfeeding. Penicillins have one of the best safety records in pregnancy, and benzathine penicillin is the required treatment for syphilis in pregnancy.
- Have heart failure or are on a salt-restricted diet — some injectable forms carry a sodium load.
Practical points:
- Take penicillin V on an empty stomach — about an hour before food or two hours after. Food reduces absorption significantly.
- Space doses evenly, usually four times a day.
- Complete the course, particularly for strep throat.
- Keep every appointment for benzathine injections. Missing one leaves you unprotected.
Possible cautions and when to ask a pharmacist or doctor
Common effects are nausea, diarrhoea, stomach discomfort and rash. Mild diarrhoea is usual with antibiotics.
Get emergency help for:
- Swelling of the face, lips or tongue, difficulty breathing, widespread hives, dizziness or collapse — anaphylaxis. This is the reason penicillin injections are given where someone can respond.
- A rash with blistering, peeling, or sores in the mouth or eyes.
- Severe, watery or bloody diarrhoea with fever or cramping, during or in the weeks after a course — possible Clostridioides difficile infection.
Two reactions specific to certain uses:
After a syphilis injection, some people develop fever, chills, headache and muscle aches within hours — the Jarisch-Herxheimer reaction, caused by bacteria dying rather than by the medicine. It settles within a day. It is worth knowing about, and worth mentioning to the clinic if it happens.
Penicillin injections must not be given into a vein in the depot forms, and the injection itself can be painful — that is expected.
Contact your doctor if the infection is not improving after two to three days, if fever climbs, if a sore throat is worsening or you cannot swallow, or if you have missed a benzathine injection — that appointment should be rebooked immediately rather than waiting for the next one.
Related medicine guides
Frequently asked questions
What is penicillin used for? Streptococcal throat infection, dental and skin infections, syphilis, preventing rheumatic fever recurrence, and serious infections such as meningitis and endocarditis.
What is a penicillin injection used for? Depending on the form: syphilis (a single benzathine injection), preventing rheumatic fever (an injection every three to four weeks), and serious hospital infections (high-dose benzylpenicillin).
Why can’t I just take a tablet for syphilis? Benzathine penicillin injection is the recommended treatment and the only one recommended in pregnancy. Oral alternatives are not considered equivalent.
Will penicillin treat flu or a cold? No. Those are viral, and antibiotics have no effect on them. Most sore throats are also viral.
Why must I take it on an empty stomach? Food significantly reduces absorption of penicillin V. Take it about an hour before eating or two hours after.
I was told as a child I’m allergic — is that permanent? Not necessarily. Many childhood allergy labels turn out to be wrong, and carrying one leads to worse antibiotic choices. Ask about being formally assessed.
Why do I need injections for years after rheumatic fever? To prevent another streptococcal infection triggering a recurrence, which causes further heart valve damage. Missing appointments removes that protection.
Can I stop when my throat feels better? No. The full ten-day course is what prevents rheumatic fever.