Hi. Thanks for your query. Noted the history of your 18 month old son having vomiting with milk you normally buy, vomits curdled milk. Not with other brands. This means that there is an intolerance to something in the milk that you buy normally that the other brands do not have. I would advise you the following in such a situation: Check the stool for lactose intolerance. Never give the milk and milk products that do not suit your son. Prevention is better than cure as he can develop even more problems like loose stool, mal-absorption, dislike for certain foods and beverages and may create problems for growth. Consult your Pediatrician for further clinical evaluation, and solutions.