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Cellulitis is an infection of the skin and the subcutaneous tissue, or the tissue directly under the skin. Cellulitis causes redness, swelling, and pain at the affected site. Usually the causative microorganism cannot be identified. Our skin is covered by microorganisms and any break in the skin, which is a protective barrier, allows these microorganisms to get into and under the skin and cause an infection. At any rate, research has shown that most cases of cellulitis are caused by B-hemolytic strep, the same strep that causes throat infections. Annual U.S. ambulatory visits for skin and soft tissue infections increased from 4.6 million in 1997 to 9.6 million in 2006 with the largest relative increase occurring in ERs. It is during this period that the dreaded “mersa,” or methicillin-resistant staph aureus (MRSA), reared its ugly head. During this period prescriptions for antibiotics with activity against MRSA became increasingly prescribed with trimethoprim-sulfamethoxazole (Bactrim, a sulfa-containing antibiotic) accounting for most of this increase. But Bactrim has poor activity against other bacteria like strep so two antibiotics needed to be prescribed. In most cases Bactrim was added “just in case.”

Some experts are wondering whether the “just in case” pendulum has swung too far. A few months ago, a study was published that attempted to answer the question of whether prescribing two antibiotics as opposed to just one (i.e. cephalexin, or Keflex) was more effective at clearing the infection. The study looked at people with non-purulent infections, which means a skin infection without an abscess, who presented to five different ERs. Study participants were randomly assigned to receive a seven-day course of cephalexin or a seven-day course of cephalexin plus Bactrim. The study found no difference between the two groups in terms of clearance of the infection. There were a small number of treatment failures and more than half of these failures subsequently developed an abscess. The study’s authors did note that in the group receiving two antibiotics there were more side effects, mostly stomach upset and diarrhea.

The results of this study indicate that most patients presenting with non-purulent cellulitis without abscess can successfully be treated with one antibiotic as the causative agent is usually strep. In addition those who present with a purulent (pus-producing) cellulitis and/or abscess are much more likely to have MRSA or MSSA (methicillin-sensitive) as the causative agent and need to be treated with two or more antibiotics. In general, the less medication the patient receives, the better off he or she will be, often because of side effects of the medicines. For example, sulfonamides, of which Bactrim is one, have a fairly high incidence of moderate to severe allergic reactions due (to the sulfa it contains). In short, the addition of a second antibiotic “just in case” may often do more harm than good and should be avoided unless the situation warrants it.

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