What Should Be Done About Accidental Ingestion Of A Higher Dosage Of Piriton CS Than Prescribed Followed By Vomiting By A Child?

Hi, my son is 6 yrs old and doctor has prescribed him 2.5ml of caugh syrup Piriton CS, but accidentally i have given him 5 ml..I am worried now...he has vomitted 20 mins after the medication...please suggest wat to do....he is sleeping currently and temparature is normal.