Background: Cow´s milk allergy is the most common cause of food allergy in young children. Ingestion of milk products in children with a milk protein allergy can lead to anaphylaxis and must be avoided. Most guidelines generally recommend the use of an extensively hydrolyzed formula (EHF) in these cases; however rare allergic reactions can still occur. Here, we present a 3-month-old who developed anaphylaxis to a cow’s milk formula. Subsequently he developed a rare systemic reaction to soy and an extensively hydrolyzed formula. Case: The patient had an unremarkable past medical history and presented with signs and symptoms consistent with anaphylaxis after being fed cow’s milk formula for the first time. Symptoms included immediate vomiting, wheezing, stridor, angioedema of eyelids and lips. Although IM epinephrine was given, the patient continued to clinically deteriorate becoming more lethargic and necessitating admission to the pediatric intensive care unit. Subsequently, a trial of soy formula ingestion reproduced similar symptoms and an EHF was given. However immediately after taking an EHF, he developed facial angioedema and diffuse urticarial lesions. Conclusion: In most cases with a cow’s milk allergy, an extensively based formula can be tolerated safely due to a hydrolyzed protein chain. However, medical providers must be vigilant when switching formula since a rare systemic allergic reaction to EHF can still occur.