Hi It is unlikely an allergic reaction to Milk of magnesia. If it is so it would not have been limited to tonsils alone but it would have appeared on lips and/or mouth. Secondly lymph glands would generally not get swollen. It is most probably due to infection either viral or bacterial. Whether an associated sore throat, difficulty or pain on swallowing, fever are there? You have written it is 4th day and did not mention of other throat symptoms or treatment given for that. So I assume that is not severe or is subsiding. In that case it could be viral in origin. Then probably nothing else need be done. If neck glands are quite big a viral infection called Infectious Mononucleosis need to be ruled out In case if she has above mentioned symptoms you need to do the following: A swab can be taken from patch of tonsil for culture and sensitivity test ( by a doctor) Plenty of warm oral fluids (water milk etc) Paracetamol if fever is there Antibiotic may be needed if symptoms are increasing. In that case better show to an ENT specialist Take care