Cellulitis. prescription 2 antibiotics-keflex 4x day /7days & bactrim (sulfamethoxazole and trimethoprim) 2x day/7 days. after 2 days, had bad side fx-doc says stop keflex only. is this a concern?

YES: You do not indicate what the reaction was that prompted a change in therapy, but if this was an allergic reaction it was far more likely to have been due to the Bactrim (sulfamethoxazole and trimethoprim) which contains a sulfonamide.

Antibiotic choice : I would be unable to answer this appropriately. Of course, you could have reacted to either antibiotic. Of course, the physician may already know what bacteria you have and made a choice to change your antibiotic. Careful followup and communication are most important for you over the next week. Did your symptoms improve with the removal of the bactrim (sulfamethoxazole and trimethoprim)?

What's the cause: Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender) . It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.

Need to be tested.: You could have an allergy to any antibiotic. Discuss you allergies with your physician. An allergy test may help calm your fears.