Keflex (cephalexin) is typically taken as 250 mg every 6 hours or 500 mg every 12 hours, with or without food, for 7 to 14 days depending on the infection. It’s a cephalosporin antibiotic that kills bacteria, and getting the timing and duration right makes a real difference in how well it works.
Standard Adult Dosing Schedule
The usual adult dose is 250 mg taken four times a day (every 6 hours). An alternative schedule of 500 mg every 12 hours works for many infections too, which is easier to fit into a normal routine. Your prescription label will tell you which schedule your doctor chose.
For more severe infections, the daily dose can go up to 4 grams, split into two to four doses throughout the day. Skin infections, urinary tract infections, and respiratory infections all fall within this dosing range, though your doctor may adjust based on the severity and location of the infection.
Taking It With or Without Food
Keflex works whether you take it on an empty stomach or with a meal. If it causes nausea or stomach discomfort, taking it with food helps. Many people find that eating a small snack or meal alongside their dose prevents the digestive side effects entirely.
What to Do if You Miss a Dose
If you realize you missed a dose and it’s still well before your next scheduled one, take it as soon as you remember. If it’s almost time for your next dose, skip the missed one and continue your regular schedule. Don’t double up to make up for it.
Keeping doses evenly spaced matters more than the exact clock time. The goal is to maintain a steady level of the drug in your system so bacteria can’t recover between doses. Setting a phone alarm for each dose helps, especially with a four-times-daily schedule.
Finishing the Full Course
Keflex is typically prescribed for 7 to 14 days. For strep throat and other streptococcal infections, a minimum of 10 days is recommended even if you feel better sooner. Stopping early allows surviving bacteria to regrow, potentially causing a relapse or contributing to antibiotic resistance.
Feeling better after a few days is normal and expected. That improvement means the drug is working, not that the infection is fully cleared. Continue taking every dose until the prescription is finished.
Giving Keflex to Children
Children over 1 year old receive a liquid suspension dosed by weight. The standard range is 25 to 50 mg per kilogram of body weight per day, divided into equal doses. For ear infections, the dose is higher: 75 to 100 mg per kilogram daily. The liquid form should be stored in the refrigerator and thrown away after 14 days, even if there’s medication left in the bottle.
Shaking the bottle well before each dose ensures the medication is evenly mixed. Using the measuring syringe or cup that came with the prescription gives a more accurate dose than a household spoon.
Common Side Effects
The most frequent side effects are digestive: diarrhea, nausea, stomach cramps, and gas. These are usually mild and improve as your body adjusts or once you finish the course. Taking Keflex with food often reduces these symptoms.
Watch for signs that diarrhea has crossed into something more serious. An antibiotic-associated intestinal infection called C. diff can develop during or up to three months after a course of antibiotics. Contact your doctor if you experience more than two watery stools a day, blood in your stool, fever, or severe abdominal pain. These symptoms are uncommon but need prompt attention.
Drug Interactions to Know About
Probenecid, a medication used for gout, can raise Keflex levels in your blood. This increases the risk of side effects like nausea, vomiting, diarrhea, and kidney problems. The combination is especially concerning at high Keflex doses or for people with existing kidney issues.
Let your prescriber know about all medications you’re taking, including over-the-counter drugs and supplements, before starting Keflex.
Penicillin Allergy and Keflex
Keflex is a first-generation cephalosporin, and there is some chemical overlap with penicillin. Cross-reactivity rates for first- and second-generation cephalosporins in people with penicillin allergies range from 1% to 8%. That’s low but not zero. If you’ve had a serious allergic reaction to penicillin (hives, throat swelling, anaphylaxis), your doctor will weigh this risk before prescribing Keflex. A mild side effect from penicillin, like an upset stomach, is different from a true allergy and generally doesn’t pose a concern.
Storing Capsules and Liquid Forms
Capsules and tablets can be stored at room temperature, away from moisture and direct light. The liquid suspension has stricter requirements: keep it refrigerated and discard it 14 days after the pharmacy prepared it. Even if medication remains in the bottle after that window, it may no longer be effective or safe to use.