Law enforcement and veterinary officials are planning an outreach campaign to educate veterinarians about a new frontier in the opioid epidemic: people so desperate for drugs that they take medication that had been prescribed to pets. “The misuse of pet medication has serious safety implications — for people and animals,” said Middlesex District Attorney Marian T. Ryan, in a letter that will be printed in the Massachusetts Veterinary Medical Association newsletter this week. “Educating people about the signs of drug misuse, available treatment resources and how to properly store and dispose of all medications is a crucial part of helping to stem the tide of overdoses and death.”