Don't let the doctor get away with this! It should not be necessary for an infant to be taking Zantac! Further, many children who develop or have developed austic disorders have gastrointestinal problems (acid reflux)! And an infant or young child has no way to tell us what his problem is. This could be serious. Get another doctor; do the research; and seriously consider not having the infant further immunized at this point. But do the research on all of these issues.