 [ Symptoms ] if my son finished his treatment for strep and i was diagnosed the next day is he susceptible to it  my 2 year old son finished his amoxicillin treatment on tuesday and i was diagnosed with strep throat on wednesday how long does the amoxicillin stay in a persons system [ Disease ] not long less than a day before all traces of amoxicillin are gone so there are not post treatment protective effects your son is susceptible to getting strep again i hope that you were both properly lab tested to confirm strep