Hello, Your brother-in-law is having an allergic reaction to this antibiotic. Has he taken Amoxicillin previously? As long as he has no problem breathing, he is not wheezing (high-pitched whistling sounds when you put your ear to his chest and back), and there is no swelling of his throat, tongue, lips, face, or neck then it will be safe to watch him at home until his hives and swelling resolve. Make certain that his medical records both at home and at his doctor s office have a label on them stating he is allergic to Amoxicillin. Wearing a wrist or chest notation warning about his allergy would be a good idea, too. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Arnold Zedd, Pediatrician