Sulfameth trimethoprim is bactrim,a sulfa drug. "Sulfa allergy" is a term used to describe adverse drug reactions to sulfonamides, a group of drugs that includes those with and without antibiotic characteristics. Common sulfa antibiotics include Septra, Bactrim and Pediazole.
The duration of sulfa reactions depends on how long the drug was in your system prior to taking benedryl, and how allergic you are.It also depends on the state of your immunity as to whether you are having a state of lowered immunity like in pregnancy,stress,after surgeries,burns,having autoimmune or other immune related diseases. The time duration and severity of an allergic reaction is variable and cannot be predicted. Sometimes drug allergies clear up in hours and sometimes in weeks or months.There is no method to predict this.
To avoid sulfa drug allergy in future,pls avoid taking any sulfa drug and before purchasing any drug,tell the pharmacist about the prior drug reaction to sulfa.For now, You could take some antiallergic medications like benadryl or laritin and apply calamine lotion at the site and keep the area clean. If the symptoms worsen,pls consult your doctor as soon as possible.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any other queries.
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