It will take a day or two, just watch for itching which can sometimes happen but its usually pretty mild.

I have a DD who is allergic to any form of penicillin so make sure the Dr has it written on his file.
It also doesn't hurt to remind the Dr when you need more medication (learnt that from experience) as you don't want to be spending extra money for a script that should never have been given.

If this is the correct script you will notice a difference in about 36 hours.