Is cephalexin a strong antibiotic?

Your ear has been throbbing for days. Perhaps you have a hacking cough that just won’t go away. Or, you experience a burning sensation when you urinate. One trip to the doctor and a diagnosis later, you’re walking out of the office with a prescription for cephalexin in hand. 

Cephalexin (also known by the brand name Keflex) is an antibiotic from the cephalosporin family that is approved by the Food and Drug Administration (FDA) to treat a number of bacterial infections, including skin and tissue infections, respiratory tract infections, urinary tract infections (UTIs), and ear infections. 

This drug is often favored by healthcare providers because it is typically “well-tolerated” by patients and has less toxicity than other classes of antibiotics, such as fluoroquinolones, according to Hannah Imlay, MD, an assistant professor of infectious diseases at the University of Utah Health in Salt Lake City. All that said, how strong is cephalexin? And how quickly will it work to clear the infection?

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Is cephalexin a strong antibiotic?

Cephalexin is a first-generation cephalosporin and broad-spectrum antibiotic that is effective against a number of pathogens including Streptococcus pneumonia, Staphylococcus aureus, and Streptococcus pyogenes. These bacteria cause common ailments, including middle ear infections (otitis media), genitourinary tract infections, skin infections, soft tissue infections, respiratory infections, and bone infections.

However, there are some notable exceptions. “It doesn’t cover some stronger bugs, like MRSA [Methicillin-resistant Staphylococcus aureus] or some stronger staph infections or pseudomonas,” says Greg Castelli, Pharm.D., director of academic and clinical pharmacy in the Department of Family Medicine at the University of Pittsburgh School of Medicine.

Your healthcare professional may take a culture to ensure that your infection is caused by a type of bacteria that is susceptible to cephalexin. Like other antibiotics, cephalexin does not help treat the common cold, flu, COVID-19, or other types of viral infections. 

How quickly does cephalexin work?

Cephalexin starts working within one hour of taking the first dose. However, it takes a little bit longer to notice the difference in your symptoms, which is in indication that the bacterial infection is going away. This prescription drug works by inhibiting the growth of bacteria, according to Dr. Castelli. Depending on the type of infection you have, you may start feeling better as soon as the next day after starting treatment. Other more severe infections may take longer. 

If you’re not feeling better after the entire course of your antibiotic, you should call your healthcare provider. And even if you do start feeling better before you’ve finished your prescription, it’s important to take the entire course of the antibiotic to ensure the infection is gone and to prevent antibiotic resistance. 

How is cephalexin used?

Cephalexin comes in three oral dosage forms: capsule, tablet, and liquid. Dosage depends on age. For children ages 1 year to 15 years old, the dose is based on body weight. For adults, the typical dose is between 250 mg to 500 mg, taken up to four times a day (though it might be as low as twice a day), for seven to 14 days. 

Consistent dosing is particularly important for cephalexin. “You have to keep a certain amount of the drug in your body to consistently have that bacteria-killing effect,” Dr. Castelli explains. “For most infections, four times a day is what you need.” 

In the case of a missed dose, if only a couple of hours have passed, it’s okay to take it when you remember, according to Dr. Castelli. But if you’re close to your next dose, simply wait until then and resume normal dosing. Do not double up on medication to try to make up for a missed dose. 

Cephalexin can be taken with or without food. If you experience abdominal pain or any stomach problems while on the drug (which is common for many antibiotics), take it with a light snack, such as yogurt, which will also provide probiotic benefits to help regulate your gut bacteria. 

Side effects of cephalexin

Common side effects of cephalexin include:

• Nausea
• Vomiting
• Diarrhea
• Yeast infection (either vaginal or oral)
• Dizziness
• Fatigue
• Headache

If you’re experiencing heavy diarrhea (five or more times a day), Dr. Imlay recommends contacting your healthcare provider, as that could be a sign of a Clostridioides difficile infection (also known as C. diff). Other symptoms may include stomach pain, fever, and bloody or watery stools. A C. diff infection can cause colitis and damage the colon, so it’s important to get quick treatment. These infections can occur while taking an antibiotic, or even up to several months after finishing the antibiotic.

If you experience any of the following adverse effects, which may indicate an allergic reaction or hypersensitivity to cephalexin, you should seek emergency medical attention immediately:

• Skin rash or hives
• Swelling of the face or mouth
• Trouble breathing or swallowing

If you’re worried about any of your side effects or if you are still not feeling well after you’ve taken the entire prescription, contact your healthcare provider.

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What should I avoid while taking cephalexin?

While cephalexin has fewer drug interactions than other antibiotics, there are a handful to be aware of. Tell your healthcare provider about all the medications you take, including prescription drugs, over-the-counter medicines, and vitamins or dietary supplements before taking cephalexin. Some interactions include:

• Metformin: Co-administration of the Type 2 diabetes drug and cephalexin resulted in increased plasma concentration and decreased renal clearance of metformin, according to one study. This means that metformin levels could build up in the body, which could increase the drug’s blood sugar-lowering effects and/or side effects.
• Probenecid: The uric-acid reducer inhibits the renal excretions of cephalexin, meaning that cephalexin levels in the body can increase, potentially causing worsened side effects. 
• Warfarin: Cephalexin can affect your levels while on blood thinners, increasing the risk of bleeding. “The reason is that the concentration of blood thinners depends on the bacteria in your gut,” Dr. Imlay says. 
• Zinc: Studies have found that this supplement may reduce the absorption rate of cephalexin. It’s advised that you allow at least three hours to pass after a cephalexin dose before taking zinc. 

You should also alert your healthcare provider if you have renal issues, such as kidney disease, as they may change your dosage, says Dr. Castelli. “Antibiotics can tend to hang out longer [in your body] if you have kidney dysfunction,” he says.

Caution should be taken if you are breastfeeding, as cephalexin will be present in your breast milk. (There aren’t adequate studies on the effect of cephalexin in pregnant women, so it’s advised to take it only when necessary.) Always check with your provider if you are pregnant, planning to become pregnant, or breastfeeding.

Cephalexin should not be taken if you are allergic to cephalosporin antibiotics. In addition, seek medical advice if you are allergic to penicillin. In some cases, cephalexin is an alternative option if a patient is allergic to penicillin. In other cases, because the two families of drugs are closely related, there is also a chance of cross-reactivity (i.e., being allergic to both drugs.)

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