risk management

I found a mistake of dispensed medicine when I was checking the stock.
somebody picked up a different medicine and handed to a patient.

we told the doctor about the mistake. he said it was no big deal.
then we called the patient. he said he had thought the medicine was different
but had already taken some. no side effect had occured.

we apologized the patient and went to his house and replaced the medicine.
we were relieved it turned out to be nothing serious.

How can we avoid mistakes? we have to think about it every day.