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By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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Cephalexin (Keflex)

Brand Name: Keflex

Available in 250 mg, 333 mg, 500 mg, 700 mg, and 750 mg capsules; oral suspension;  75 mg, 150 mg, 300 mg, 600 mg chewable tablets; and 250mg and 500mg non-chewable tablets

History and Background

Thanks to work by Alexander Fleming (1881-1955), Howard Florey (1898-1968) and Ernst Chain (1906-1979), penicillin was first produced on a large scale for human use in 1943. At that time, the development of a pill that could reliably kill bacteria was remarkable and many lives were saved that surely would have been lost without it.

But quickly, it became obvious that this new "wonder drug" could bear improvement. For example:

  • Penicillin is not well absorbed from the intestinal tract meaning that at least 70% of an oral dose is wasted.
  • Penicillin is also a short-acting medication, with half of the amount circulating being removed from the body every half hour.
  • Not all bacteria have the type of cell wall which is susceptible to destruction by penicillin. (Bacteria are classified as Gram negative or Gram positive, depending on the cell wall characteristics. Penicillin is able to punch holes through the Gram positive cell wall but is not very effective against the Gram negative cell wall.)
  • Staphylococci (an important group of bacteria) have developed an enzyme to break the penicillin molecule apart.

The cephalosporin class was developed to improve upon the accomplishments of the penicillin class. Like penicillin, the cephalosporins are biological in origin. Cephalosporins stem from a substance called cephalosporin C that is produced by the bacterium Cephalosporium acremonium

Cephalosporin antibiotics are classified into three groups. The first group developed (the so-called first generation cephalosporins) is effective against most Gram positive infections, some Gram negative infections, and is able to withstand the anti-penicillin enzymes produced by Staphylococci. Most anaerobic infections are also sensitive to the first generation cephalosporins. The second and third generation cephalosporins are sequentially more effective on resistant Gram negative and anaerobic bacteria. They are generally reserved for more serious infections and/or human use. Cephalexin is a first generation cephalosporin.

How This Medicine Is Used

Cephalexin is a good broad spectrum antibiotic, which means it is useful in many types of uncomplicated infections. It is especially useful against Staphylococcal infections (most skin infections) and is commonly used for long (6-8 week) courses against deep skin infections (pyodermas) or kidney infections (pyelonephritis) without concern of adverse reaction.

Side Effects

Nausea may be seen in some individuals receiving cephalexin. In general, this problem is solved by giving the medication with food.

Occasionally cats will develop a fever in response to cephalexin. If this occurs, a different antibiotic should be selected. Inform your veterinarian.

Occasionally dogs will develop hyperexcitability and drooling in response to taking cephalexin. If this occurs, another antibiotic should be selected. Inform your veterinarian.

Interactions with other Drugs

In treating more serious infections, cephalsporins are often used in combination with other antibiotics to cover a broader group of bacteria when a specific agent of infection is not known.

Concerns and Cautions

This medication is commonly used for several months without monitoring tests of any kind. It is felt to be safe for long-term use.

The oral suspension is only good for a 2-week period after it has been reconstituted and must be refrigerated. Capsules and tablets should be stored at room temperature away from light.

Cephalexin does not cross into milk well and thus is probably not a concern in lactating females. It does, however, cross the placenta and is best not used in pregnancy if it can be avoided.

The use of cephalosporin antibiotics can turns some urine dipsticks falsely positive for glucose.

If a dose is skipped, do not double up on the next dose.

Should nausea result, give it with food.

It is our policy not to give dosing information over the Internet.


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