I have also been in the situation of making a drug error. It is an awful experience to go through, but i immediately informed SHO, together made a plan of treatment required, informed the patient, completed incident form and handed it personally to my manager. I kept everything to the facts. My manager was great she supported me through out and thanked me for having been honest. I actually had a collegue express surprise that i was reporting it which i found hard to believe. As it happens the patient suffered no ill effect from my error and again was greatful to know that it was being dealt with. Sometimes it is just a case of being more careful and taking more time even when in a rush. It can be a hard lesson, but make a mistake once and you will do everything not to be in that position again.