Thank you for your query. It must be difficult seeing your little go through this tough period. I completely understand. Would you please clarify a few things for me ? 1)How is your son s overall condition , other than the fever? Is he eating better, playing, having some amount of normalcy? or is he cranky, irritable, lethargic? 2)Does he have any other symptoms other than fever (cough-dry or productive, cold, headaches, digestive problems, urinary symptoms, swollen glands on neck /under chin/under jaw, does it hurt to swallow)? 3)Did his pediatrician see him before changing the antibiotic? Tonsils are the watchmen that protect the body from infection that crosses though the nose or mouth. They take most of the blow and brunt when an infection occurs. This prevents it from entering out body ,thus preventing severe disease in most cases. Tonsillitis in the inflammation of the tonsils caused due to infection by bacteria or viruses. Viral tonsillitis is more common and will resolve itself in normal individuals without treatment (symptomatic treatment may be taken to ease through). This maybe accompanied by a fever, cold, cough running nose, throat ache but may not elicit a systemic (body) response , such as increased Leuckocytes( cells that fight infection). These infections find it easier to attack an individual when they are already weak (from a bacterial infection, for instance). Bacterial tonsillitis (Strep. throat, for instance) is associated with similar symptoms and also with a systemic response. After two days of antibiotic therapy the bacterial infection and its systemic response begin to regress. This is an opportune moment for the virus to kick in. Unfortunately, antibiotics don t work against virus. This may be the scenario with your son. Certain complications of a bacterial tonsillitis - peritonsillar abscess ( a collection of pus around the tonsils) can cause persistent fever. I recommend seeing his pediatrician ( if you haven t already done so) If there are no complication from the bacterial tonsillitis, and no localizing signs (evidence) of a Urinary tract infection, diarrhoeal disease, meningitis, a viral infection is most likely. This will settle on its own and the mainstay of treatment is rest, hydration, more rest, proper diet, and more hydration. Fever medications can be given as prescribed. The full course of the antibiotics will need to be completed. If he seems otherwise fine, dont worry too much about the fever. Act to prevent its complications like dehydration Monitor the quantity and colour of the urine. it should be a light yellow in colour (except for the first urine of the day which may be dark as it is concentrated through the night). check his tongue. it should be moist all the time. Fatigue is expected. Lethargy (inability to even walk on his own), excessive daytime drowsiness or altered sensorium, seizures, low urine output, loss of weight more than 5% of his current body weight, fever that does not reduce with medications should caution you for getting medical help right away (ER) Drug fever is a rare event. Certain antibiotics elicit fever as a response to the antibiotic. In the absence of other serious allergic symptoms, the medicine can be continued if it is necessary to treat the infection. Once the medicine is stopped, the fever subsides. Children are resilient and will bounce back in no time. They make everything obvious which makes it easy for watchful parents to monitor. The minute they start to feel better, they ll be back to playing, an improvement in eating and longer waking hours can be observed. If something is wrong, they wont eat or want to be held and will cry incessantly. Please follow your instincts. You know him best. Hope I ve answered your query. Wish you both good health. Please get back to us if you need any clarification