Cephalexin, sold as Keflex since 1969, is one of the most widely prescribed pills for common skin, throat, and bladder infections.1Wikipedia. Cefalexin It kills bacteria outright rather than just stopping their growth, by attacking a structure human cells don’t have. Its strength is narrow by design, aimed mainly at the bacteria that live on skin and in the throat rather than those from the gut. And because the body absorbs and clears it fast, how you take it matters almost as much as what it targets.
How Cephalexin Kills Bacteria
Doctors call cephalexin a first-generation cephalosporin, meaning one of the earliest members of a drug family related to penicillin.2DrugBank. Cephalexin Like penicillin, it contains a ring-shaped core called a beta-lactam, and that ring is the part that does the work.3DrugBank. Cephalexin It latches onto bacterial proteins that cross-link the strands of the cell wall.4Synapse. What is the mechanism of Cephalexin? New wall then gets built without its cross-braces, and it grows weaker as the bacterium tries to grow.5Synapse. What is the mechanism of Cephalexin? Eventually the cell can no longer hold its shape, and it bursts open and dies.6Synapse. What is the mechanism of Cephalexin? Human cells have no cell wall, so the drug leaves your own cells alone.
Because the drug disrupts wall building, it acts on bacteria that are actively dividing. Steady levels across the day keep catching bacteria as they divide, while missed doses give survivors time to rebuild.
The structure around the ring was changed in cephalexin so it survives stomach acid and can be taken by mouth. Its resemblance to penicillin is also why allergy questions come up, which is covered later.
More is not always better. In lab tests against staph and strep, cephalexin worked best at a middle concentration and slightly less well when the amount was pushed higher.7PubMed. Biophotometric comparison of cephalexin and cephradine against gram-positive bacteria Taking extra pills won’t speed recovery.
Which Bacteria Cephalexin Works Against
Bacteria fall into two broad groups based on how their walls are built. Gram-positive bacteria include most skin and throat germs, and gram-negative bacteria include many gut germs. Older cephalosporins like cephalexin are strongest against gram-positive bacteria, especially staph and most strep.8Kirk-Othmer Encyclopedia of Chemical Technology. Cephalosporins Their activity against gram-negative bacteria is only modest.9PubMed Central. Updates in the Management of Cephalosporin-Resistant Gram-Negative Bacteria
In lab testing, E. coli was only partly sensitive to cephalexin and Serratia was resistant.10Asian Journal of Pharmaceutical and Clinical Research. EVALUATION OF BACTERIAL RESISTANCE OF CEFADROXIL, CEPHALEXIN AGAINST GRAM-POSITIVE AND GRAM-NEGATIVE MICRO-ORGANISMS Pseudomonas, a hard-to-treat germ, was also only partly sensitive.11Asian Journal of Pharmaceutical and Clinical Research. EVALUATION OF BACTERIAL RESISTANCE OF CEFADROXIL, CEPHALEXIN AGAINST GRAM-POSITIVE AND GRAM-NEGATIVE MICRO-ORGANISMS That makes cephalexin a good first choice for typical skin infections but not for every urinary or hospital infection.
When a swab or urine culture is sent to a lab, it names the bacteria and tests which drugs work against them. On the report, “sensitive” means cephalexin should work and “resistant” means it likely won’t. “Intermediate” means borderline, and your doctor may switch drugs.
Cephalexin also doesn’t cover MRSA, a staph that has changed its wall proteins. Cephalexin can’t bind those altered proteins well. A skin infection that keeps spreading on cephalexin may be MRSA and needs a recheck.
Infections Cephalexin Treats and How Well It Works
For mild to moderate skin infections, cephalexin cured about 87 of 100 people and cleared the bacteria in about 89 of 100.12PubMed. Cefdinir versus cephalexin for the treatment of skin and skin-structure infections For staph skin infections, twice-daily cephalexin worked as well as a four-times-daily alternative, with treatment failures rare.13PubMed. Treatment of staphylococcal skin infections: a comparison of cephalexin and dicloxacillin The few repeat infections were in children with blistering impetigo.14PubMed. Treatment of staphylococcal skin infections: a comparison of cephalexin and dicloxacillin In impetigo, sores usually cleared within the first week.15PubMed. Treatment of staphylococcal skin infections: a comparison of cephalexin and dicloxacillin
Cephalexin is also commonly used for urinary tract infections.16PubMed. Cefdinir vs cephalexin for the treatment of urinary tract infections: A retrospective evaluation In women with uncomplicated UTIs, taking it twice a day failed no more often than four times a day, about 13 versus 17 in 100.17PubMed Central. Two Times Versus Four Times Daily Cephalexin Dosing for the Treatment of Uncomplicated Urinary Tract Infections in Females For strep throat and simple bladder infections, the daily dose can be split into two doses 12 hours apart.18Health Canada. CEPHALEXIN Product Monograph Strep infections, though, need at least 10 days of treatment.19Health Canada. CEPHALEXIN Product Monograph
Many people feel better within two to three days. Skin redness may take several days to shrink. If redness worsens, a fever appears, or the infection keeps spreading after two to three days, call your prescriber.
Cephalexin does nothing for colds, flu, or other viral infections. Most sore throats are viral, and a confirmed strep test is what justifies it.
The right length of treatment depends on the infection, not the drug. Strep throat has a firm 10-day minimum, while a simple bladder infection is usually treated for a much shorter course, so don’t judge your prescription by a friend’s. Skin infections are often treated for around a week, and the prescriber may extend that if the area hasn’t clearly improved, based on how the skin looks at a recheck. The schedule on your label was chosen for your infection, so don’t switch between twice-daily and four-times-daily on your own.
How Your Body Absorbs and Clears Cephalexin
Nearly all of a dose is absorbed, with about 9 in 10 recovered in urine within a day.20PubMed. Cephalexin pharmacokinetics in patients with cystic fibrosis The gut uses a special transporter to pull the drug in, but absorption stays high even without it.21PubMed. Quantitative evaluation of PEPT1 contribution to oral absorption of cephalexin in rats Only about 10% of the drug sticks to blood proteins.22PubMed Central. Dermal Drug Levels of Antibiotic (Cephalexin) Determined by Electroporation and Transcutaneous Sampling (ETS) Technique Most of it is therefore free to move out of the blood into skin and other tissues, which suits skin infections.23PubMed Central. Dermal Drug Levels of Antibiotic (Cephalexin) Determined by Electroporation and Transcutaneous Sampling (ETS) Technique
The exit is fast. In healthy adults, the level in the blood halves in about an hour,24PubMed. Cephalexin pharmacokinetics in patients with cystic fibrosis so levels fall quickly between doses. The kidneys concentrate the drug in urine,25PubMed. Pharmacokinetics and tissue fluid distribution of cephalexin in the horse after oral and i.v. administration which makes urine levels high and helps against bladder infections. How fast people clear it can vary threefold from person to person, though.26PubMed. Cephalexin pharmacokinetics in patients with cystic fibrosis
With a half-life that short, one daily dose would leave long stretches with little drug on board. Spacing doses evenly keeps levels useful. If you miss a dose, take it when you remember unless the next one is close. Don’t double up.
You can take cephalexin with or without food. Food can ease nausea. Mild diarrhea is common, but severe or bloody diarrhea needs a doctor.
Penicillin Allergy and Cephalexin
People with a true penicillin allergy rarely react to cephalosporins, especially ones built differently on the side.27PubMed. The Use of Perioperative Cephalexin in Penicillin Allergic Patients in Dermatologic Surgery: An Advisory Statement Cephalexin carries a slightly higher risk for people allergic to amoxicillin or ampicillin, though not for other penicillins.28PubMed. The Use of Perioperative Cephalexin in Penicillin Allergic Patients in Dermatologic Surgery: An Advisory Statement For people with low-risk penicillin allergies, severe reactions are extremely rare.29PubMed. The Use of Perioperative Cephalexin in Penicillin Allergic Patients in Dermatologic Surgery: An Advisory Statement So tell your prescriber exactly which drug you reacted to and how.
A useful description includes the drug name, your age at the time, and what happened. A rash years ago is very different from swelling or trouble breathing. Many childhood “allergies” were never true allergies, and allergy testing can clear an old label.
While taking cephalexin, watch for hives, itching, or a new rash. If you get a milder rash, stop the drug and call your doctor. Swelling of the lips, face, or throat, or trouble breathing, is an emergency.
When Bacteria Resist Cephalexin
Some bacteria make enzymes that break the drug’s beta-lactam ring.30BOC Sciences. Cephalexin: The First Generation Cephalosporin These enzymes treat the antibiotic like food and recycle themselves to destroy more.31PubMed Central. β-Lactam antibiotic targets and resistance mechanisms: from covalent inhibitors to substrates Older cephalosporins are especially easily broken down by many of these enzymes.32PubMed Central. beta-Lactamase stability and in vitro activity of oral cephalosporins against strains possessing well-characterized mechanisms of resistance Some common enzymes found in gut bacteria don’t break down cephalexin, though.33PubMed Central. Updates in the Management of Cephalosporin-Resistant Gram-Negative Bacteria The other escape route is for bacteria to reshape their wall-building proteins so the drug can’t latch on.34BOC Sciences. Cephalexin: The First Generation Cephalosporin
The enzymes are made by the bacterium itself, which can acquire the instructions for them from other bacteria. That is how resistance can show up in a germ that was never exposed to the drug before.
You can help limit resistance by taking every dose for the full course. Don’t save leftover pills for later illnesses, and don’t share antibiotics. It also helps to ask whether an antibiotic is truly needed.
Kidneys, Pregnancy, and Other Medicines
Your lab report may show creatinine clearance, an estimate of how well your kidneys filter, and doses depend on it:35U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
- Above 60 mL/min: no change.36U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
- 30 to 59: no change, but no more than 1 g a day.37U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
- 15 to 29: 250 mg every 8 or 12 hours.38U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
- 5 to 14, not on dialysis: 250 mg once a day.39U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
- 1 to 4, not on dialysis: 250 mg every 48 to 60 hours.40U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
Animal studies found no harm to the fetus, though human studies are lacking, so cephalexin is used in pregnancy when clearly needed.41U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP Its safety and dosing in children are established.42U.S. Food and Drug Administration. KEFLEX (cephalexin) Capsules, USP
Cephalexin raised peak metformin levels by about a third,43PubMed. Effect of cephalexin on the pharmacokinetics of metformin in healthy human volunteers because it slows the kidneys’ removal of metformin.44PubMed Central. Drug Interactions of Metformin Involving Drug Transporter Proteins Probenecid, a gout medicine, similarly slows the kidneys’ removal of cephalexin. Tell your prescriber if you take metformin or probenecid.
Tell your pharmacist about every prescription, supplement, and over-the-counter drug you use. Mention kidney disease or dialysis, and say if you are pregnant or breastfeeding.
Cephalexin Compared With Similar Antibiotics
Cefadroxil, a cousin of cephalexin, stays in the blood longer at the same dose,45PubMed Central. Comparative human oral clinical pharmacology of cefadroxil, cephalexin, and cephradine so it can be taken less often.46PubMed Central. Comparison of Pharmacological and Antimicrobial Properties of Cefadroxil and Cephalexin In children with skin infections, 95% on once-daily cefadroxil took every dose versus 65% on twice-daily cephalexin.47PubMed. Comparative evaluation of cefadroxil and cephalexin in children and adolescents with pyodermas Overall response was somewhat better with cefadroxil, 94 versus 86 in 100.48PubMed. Comparative evaluation of cefadroxil and cephalexin in children and adolescents with pyodermas Twice-daily cefdinir worked as well as four-times-daily cephalexin for skin infections.49PubMed. Cefdinir versus cephalexin for the treatment of skin and skin-structure infections
An antibiotic only works if you take it, so a drug with an easier schedule can do better in real life. The better response with cefadroxil is consistent with the gap in doses taken, though the study doesn’t prove the schedule alone explains it.
Cephalexin, on the other hand, is cheap and widely available. If you think the schedule will be hard to keep, ask your doctor about alternatives.
Cephalexin for Dogs and Cats
Vets prescribe cephalexin for skin, respiratory, and urinary infections in dogs and cats.50Plumb’s Veterinary Drug Handbook / Brief Media. Cephalexin Client Information Sheet Pet doses differ from human doses, so never give a pet your own prescription.
If the pill upsets your pet’s stomach, give it with food. Finish the full course, and call the vet for vomiting, diarrhea, or a rash.