Cephalexin: What is it? Warnings, Side Effects, Interactions, Dosage and Conservation

It is sometimes called first-generation cephalosporin because it was one of the first cephalosporins developed and marketed.

Cephalexin is an antibiotic of the cephalosporin family; it treats infections caused by bacteria.

Cephalexin is prescribed to treat infections of the respiratory tract, middle ear, skin, bones, and urinary tract (UTI).

It is also used to prevent infections caused by streptococcal bacteria, including the prevention of rheumatic fever. Cephalexin is not recommended for sinus infections.

It was approved by the Food and Drug Administration in 1971 and was first sold by Eli Lilly and Company under the Keflex brand; Today, Keflex is manufactured and sold by Shionogi Inc.

It is also available as a generic medicine in the form of a tablet or liquid (called a suspension).

Cephalexin and other cephalosporin antibiotics are broad-spectrum antibiotics, which means they treat many infections caused by many different bacteria.


Kill susceptible bacteria by interfering with the ability of bacteria to form cell walls, which are necessary for bacterial cells to survive.

Cephalexin should only be used when solid evidence supports its use. Excessive use of broad-spectrum antibiotics can lead to severe infections by drug-resistant bacteria ( superbugs ).

Treating colds or flu symptoms with broad-spectrum antibiotics can cause antibiotic resistance and more severe infections that are more difficult to treat.

Cefalexin warnings

If you are allergic to penicillin, there is a 10 percent chance of being allergic to Cephalexin.

You may have a higher risk of side effects with Cephalexin if you have a history of liver, kidney, or colon disease, so tell your doctor about any of these conditions.

Like other antibiotics, Cephalexin can cause an overgrowth of bacteria called Clostridium difficile in the colon. Toxins produced by C. difficile can cause diarrhea and pseudomembranous colitis.

Taking other antibiotics can also increase your risk of pseudomembranous colitis, so tell your doctor about any recent use of antibiotics.

Cephalexin and pregnancy

Cephalexin should be used only if needed during pregnancy, and caution during breastfeeding.

Tell your doctor if you are or may become pregnant or if you are breastfeeding.

Side effects

Symptoms of an allergic reaction to Cephalexin may include:

  • Eruption.
  • Swelling under the skin
  • Itch.
  • Swelling of the throat.
  • Wheezing
  • Difficult breathing

Side effects of the digestive system may include:

  • Diarrhea.
  • Abdominal pain.
  • Nausea and vomiting.

Other possible side effects of Cephalexin include:

  • Vaginal discharge
  • Headache.
  • Dizziness
  • Confusion.
  • Sores in the mouth and fatigue.

Tell your doctor about any side effects you experience.

Stop taking Cephalexin and call your doctor immediately if you experience:

  • Rash or severe inflammation of the skin.
  • Any problem breathing or swallowing
  • Sudden bruising or bleeding.


Always tell your doctor about any medications you take, including other prescription medications, over-the-counter medications, vitamins, dietary supplements, and herbal remedies.

Certain medications are known to interact with Cephalexin and can cause problems:

  • Cephalexin may increase blood levels of type 2 diabetes medication. This could increase your risk of side effects.
  • Cephalexin can accumulate in the blood when combined with the drug probenecid (Benemid, Probalan), used to treat gout.
  • In some cases, probenecid can be used together with antibiotics to increase antibiotic resistance.
  • The use of antibiotics can make contraceptive pills ineffective. The second form of contraception is recommended while taking antibiotics.

Dosing of Cefalexin

The dose of Cephalexin prescribed will depend on the type of infection and whether the person taking it is a child or an adult.

Here are general guidelines for the dosage of Cephalexin:

  • The usual dose for adults ranges between 1 and 4 grams per day, administered in divided doses.
  • Typical adult doses of Cefalexin are 250 mg every 6 hours or 500 mg every 12 hours.
  • The usual dose for a child is 25 to 50 milligrams (mg) per kilogram of weight in divided doses.
  • Keflex capsules come with 250, 333, 500 and 750 mg options.
  • Doses can be doubled for more severe infections.
  • Depending on the type of infection, the treatment can last from 7 to 14 days.

If massive doses of Cephalexin are needed, another type of cephalosporin that can be administered by injection or intravenous infusion may be used.

Here are some general rules for taking Cephalexin: take it with food or milk to avoid stomach upsets. Always take it according to the indications and for the time indicated.

Skipping doses or not finishing your prescription of Cephalexin can cause a more dangerous and resistant infection. Do not chew, split or crush Cephalexin capsules.

Take them whole with a full glass of water and food or milk.


Store the capsules in a safe place, dry, and at room temperature.

Keep the Cefalexin suspension in the refrigerator. Do not freeze. New suspension should be discarded after 14 days. If using the Cefalexin suspension, shake the liquid well before each dose.


The use of Cephalexin may interfere with some laboratory tests. These include blood tests and some urine tests for diabetes. Always tell your doctor that you are taking Cephalexin before taking the test.