Penicillin Side Effects: What to Expect and When to Call Your Doctor

Penicillin side effects are usually mild — a little stomach upset, maybe some diarrhea — and most people finish a course without any trouble. But penicillin is also one of the most common drugs people believe they’re allergic to, which makes it worth understanding the difference between an ordinary side effect, a true allergic reaction, and the kind of reaction that needs emergency care. Here’s what’s common, what’s rare, and what the research actually says about penicillin allergy.

What Is Penicillin and When Is It Prescribed?

Penicillin was the first true antibiotic, discovered from a mold in 1928, and it’s still a first-choice treatment nearly a century later for infections caused by bacteria that haven’t developed resistance to it. The version most commonly prescribed today is penicillin V potassium, taken by mouth. Doctors and dentists reach for it for strep throat, certain skin infections, and — notably for this site’s audience — dental infections and abscesses, where it’s one of two first-line options alongside amoxicillin. It works by interfering with bacteria’s ability to build their outer cell wall, which causes the bacteria to die off rather than simply stop multiplying.

Common Penicillin Side Effects

Most people who take penicillin get through their course with mild side effects, if any:

• Nausea: One of the most frequently reported side effects, especially when taken on an empty stomach.
• Diarrhea: Common and usually mild, though it’s worth monitoring for the more serious pattern described below.
• Stomach pain and upset: Reported by a meaningful share of people, often easing when the medication is taken with food.
• Vomiting: Less common than nausea alone, but can occur.
• Black or “hairy” tongue: An unusual but harmless discoloration of the tongue that can occur with penicillin and other antibiotics, resolving after the course ends.
• Vaginal yeast infection: Like other antibiotics, penicillin can disrupt the normal balance of bacteria and yeast in the body.

According to Drugs.com’s clinical reference on penicillin, the specific frequency of each side effect isn’t well quantified in the original approval studies, since penicillin has been used clinically for so long, but these remain the most consistently reported mild effects across decades of use.

Penicillin Allergy: What You Actually Need to Know

This is the single most important thing to understand about penicillin, because it affects decisions about every antibiotic a person takes for the rest of their life. According to the CDC, roughly 10% of patients in the U.S. have a penicillin allergy noted in their medical chart — but fewer than 1% of the population is truly, clinically allergic. That’s a large gap, and it matters: a penicillin allergy label that isn’t accurate can steer people toward broader-spectrum, often less effective or more expensive antibiotics for the rest of their life, which also contributes to antibiotic resistance more broadly.

Many people who believe they’re allergic actually experienced a typical side effect (like diarrhea or a mild, non-allergic rash) during childhood, or had a true allergy that has since faded — penicillin allergy can resolve over time in many people. The only way to know for sure is through allergy testing or a supervised “test dose” challenge, done by a doctor or allergist — this isn’t something to self-diagnose or dismiss on your own. If you have a documented penicillin allergy from decades ago and have never been retested, it’s worth raising with your doctor, since it may open up more treatment options if the allergy label turns out to be outdated or inaccurate.

Serious Side Effects and When They Need Emergency Care

True allergic reactions to penicillin range from mild to life-threatening, and knowing the difference matters:

• Anaphylaxis: A severe, whole-body allergic reaction involving hives, swelling of the face, lips, tongue, or throat, difficulty breathing, a rapid drop in blood pressure, and sometimes loss of consciousness. This is a medical emergency — call 911 or go to an emergency room immediately.
• Widespread hives or rash: Can be an early sign of an allergic reaction, even without the more severe features above. Stop the medication and contact your doctor promptly.
• Severe skin reactions: Rarely, penicillin has been associated with serious reactions like Stevens-Johnson syndrome, which involves blistering or peeling skin and needs emergency treatment.
• Severe or bloody diarrhea (C. diff infection): Antibiotics can disrupt normal gut bacteria and allow Clostridioides difficile (C. diff) to overgrow, causing severe, watery, or bloody diarrhea, sometimes with fever and cramping. This can develop during treatment or even weeks after finishing the antibiotic. Don’t take anti-diarrheal medication without checking with your doctor first, since slowing the gut can make a C. diff infection worse.
• Kidney effects: Penicillin is cleared through the kidneys, and in rare cases, it has been associated with a type of kidney inflammation (interstitial nephritis), which can cause reduced urination or swelling.

Penicillin for Dental Infections Specifically

Per the American Dental Association’s own clinical treatment guidelines, penicillin V potassium and amoxicillin are listed as the two first-line antibiotic options for a dental abscess that has spread beyond the local area — causing fever, facial swelling, or difficulty swallowing — or for patients who can’t have the underlying dental procedure performed right away. The typical regimen is penicillin V 500 mg taken four times daily for 3 to 7 days, with dentists reassessing after about 3 days to check that symptoms are resolving. Importantly, the ADA’s guidance is clear that antibiotics alone don’t cure a dental infection: the underlying problem (an abscessed tooth, a deep infection) still needs to be drained, treated with a root canal, or extracted. Finishing a course of penicillin without addressing the tooth itself often means the infection returns.

Who Should Be Cautious With Penicillin

Anyone with a documented, confirmed penicillin allergy should avoid it entirely unless a doctor or allergist has specifically cleared them through testing. People with significantly reduced kidney function may need a lower dose, since the drug is cleared renally. Anyone with a history of colitis or inflammatory bowel disease should mention it before starting, given the C. diff risk described above. Penicillin is generally considered one of the safer antibiotic classes during pregnancy and breastfeeding, but this should still be confirmed with your doctor rather than assumed. Older adults don’t face penicillin-specific restrictions beyond the general kidney-function consideration, though as with any new medication, reviewing your full medication list with a pharmacist is worthwhile.

Drug Interactions

Penicillin has relatively few major interactions compared to many antibiotics. A few worth knowing: it can reduce the effectiveness of hormonal birth control in some cases, so a backup method is often recommended during treatment; it may interact with methotrexate, increasing the risk of methotrexate toxicity; and high-dose probenecid (a gout medication) can raise penicillin levels in the blood, which is sometimes used intentionally in certain treatment protocols but should otherwise be flagged to your doctor. As always, give your pharmacist your complete medication list when picking up a new prescription.

Penicillin Compared to Other Dental Antibiotics

If you’ve seen other antibiotics mentioned for dental infections — amoxicillin, cephalexin, clindamycin, azithromycin, metronidazole — here’s how penicillin stacks up:

• Compared to amoxicillin: Both are first-line options and work the same way; amoxicillin is absorbed somewhat better and dosed less frequently (three times daily versus penicillin’s four), which is part of why it’s prescribed more often today, though penicillin remains an equally valid choice.
• Compared to cephalexin: Cephalexin is a related but distinct drug class (a cephalosporin) often used when a patient has a mild penicillin sensitivity that doesn’t rule out cephalosporins entirely, since there’s a small chance of cross-reactivity between the two classes.
• Compared to clindamycin: Clindamycin is typically reserved for patients with a confirmed, true penicillin allergy, partly because it carries a meaningfully higher risk of C. diff infection than penicillin.
• Compared to azithromycin and metronidazole: These are generally used as alternatives or additions when penicillin-class drugs aren’t appropriate, or to cover different types of bacteria alongside a penicillin-class drug.

Your dentist’s choice depends on your allergy history, the specific infection, and local resistance patterns — there isn’t one antibiotic that’s automatically “best” for every case.

Tips for Taking Penicillin

• Take it on an empty stomach when possible — penicillin V is absorbed best without food, though taking it with a small amount of food is a reasonable trade-off if it causes nausea.
• Finish the entire course even if you feel better after a few days; stopping early can let the infection return and contributes to antibiotic resistance.
• Space doses evenly throughout the day, since penicillin is typically dosed four times daily and maintaining a steady level matters for how well it works.
• Stay hydrated, especially if you develop any diarrhea.
• Don’t share leftover antibiotics or save them for a future infection — the dose and duration are specific to the infection being treated.

When to Call Your Doctor

Call your doctor promptly if you develop a rash or hives, if you have any trouble breathing or swallowing, facial or throat swelling, severe or bloody diarrhea, a new or worsening fever while taking the antibiotic, or signs your infection is getting worse rather than better despite several days of treatment. Any sign of a severe allergic reaction — swelling, breathing difficulty, dizziness, or a rapid heartbeat — is an emergency; call 911 rather than waiting for a callback from your doctor’s office.

Frequently Asked Questions

If I had a reaction to penicillin as a child, am I still allergic now?

Not necessarily. Many childhood penicillin allergies fade over time, and some “reactions” recorded in childhood were ordinary side effects (like a viral rash that happened to coincide with antibiotic use) rather than true allergies. Allergy testing or a supervised challenge with an allergist is the only reliable way to know your current status — don’t assume either way without checking.

Can I drink alcohol while taking penicillin?

Penicillin doesn’t have a dangerous interaction with alcohol the way metronidazole does, but alcohol can worsen some of the gastrointestinal side effects like nausea and stomach upset, and it’s generally sensible to limit it while your body is fighting an infection.

How long does it take for penicillin to start working?

Many people notice some improvement in infection symptoms within 48 to 72 hours of starting penicillin, though this varies by the type and severity of infection. Keep taking it for the full prescribed course even after you start feeling better.

Is penicillin the same as amoxicillin?

They’re closely related — amoxicillin was developed as a derivative of penicillin, with better absorption and a more convenient dosing schedule — but they’re not identical. Someone allergic to one is generally considered allergic to both, since they share the core chemical structure responsible for most penicillin allergies.

What should I do if I miss a dose of penicillin?

Take the missed dose as soon as you remember, unless it’s almost time for your next dose — in that case, skip the missed one and continue your regular schedule. Don’t double up doses to make up for a missed one.

This article is for general educational purposes and isn’t a substitute for personalized medical advice. Always follow your doctor’s or dentist’s specific guidance about diagnosis, treatment, and any symptoms that concern you.

Penicillin remains one of the most effective, well-understood antibiotics available, and for most people, a course passes with nothing worse than mild stomach upset. The bigger story is the gap between how many people believe they’re allergic and how many actually are — a gap worth closing with your doctor if your own allergy label is old or uncertain, since it can open up safer, simpler, and often cheaper treatment options down the road.