BOSTON — Dr. Virginia Sinnott has practiced veterinary medicine for 15 years, but guarding against opioid abuse by pet owners had rarely been part of her job. Until recently.

In a sign of the desperation behind the drug epidemic, Sinnott and many other veterinarians believe that more pet owners are using their cats and dogs as a ploy to obtain opioids for themselves. The warning signs are often clear and ominous.

"People become angry or enraged or belligerent far out of proportion to the level of pain their dog or cat is experiencing," said Sinnott, senior staff veterinarian in the emergency and critical care department at Angell Animal Medical Center in Boston. "They're screaming and yelling and using swear words" to demand painkillers that their pets do not need.

The problem extends far beyond Angell's busy hospitals in Jamaica Plain and Waltham. Last month, the Food and Drug Administration issued a warning to veterinarians about administering opioids, urging them to use pain-relief alternatives when possible and be vigilant for signs of misuse.

"These drugs have potentially serious risks, not just for the animal patients, but also because of their potential to lead to addiction, abuse, and overdose in humans who may divert them for their own use," FDA Commissioner Scott Gottlieb wrote.

Sinnott said she believes Angell stymies nearly all attempts to divert opioids from the hospital, but the experience has left her startled and saddened that the crisis has penetrated one of the largest veterinary hospitals in North America.

"That's the part that breaks your heart," she said. "As much as this person is screaming at you, they're suffering themselves."

Angell and its 24/7 emergency service might be more vulnerable to attempts to divert opioids than private practices, where veterinarians build close relationships with clients and their animals. But a 2016 study in Colorado, published in August by the American Journal of Public Health, showed broad concern.

The study found that 44 percent of the veterinarians surveyed were aware of opioid misuse by a client or veterinary staff member. In a striking finding, 13 percent were aware that an animal owner had intentionally injured an animal or made it sick — or made the pet seem hurt or ill — in an effort to obtain opioids.

Angell veterinarians said that they have not seen cases where animals have been intentionally injured but that three or four times a week they encounter pet owners who raise red flags for them, including knowing a suspiciously large amount of technical detail about opioids.

"It's always off-putting when someone knows more information than you would expect," Sinnott said.

Some patients ask directly, "What do I have to do to get Dilaudid?" Sinnott said. Dilaudid, the trade name for hydromorphone, is an addictive, morphine-derived painkiller that produces a heroin-like high.

The hospital does not consider itself an arm of law enforcement, except when an animal is intentionally injured, partly because suspecting that someone wants opioids for themselves or others, instead of their pets, is far from proof.

"We mostly try to stay on the veterinary side of it," Sinnott said. "But we don't prescribe or put ourselves in the position to help them commit a crime."

As a result, Angell has responded with protocols designed to head off opioid abuse and diversion.

Doses of tramadol, the most commonly prescribed opioid at Angell's pharmacy, have been cut 61 percent in recent years, to 60,326 tablets dispensed in the 12 months ended in June, compared with 153,768 tablets two years before, Angell officials said.

The hospital's response has gone further than Massachusetts regulations require, said April Mach, who directs its pharmacy, which fills 400 to 500 prescriptions a day.

Beginning in 2016, the state limited most first-time opioid prescriptions to a seven-day supply. But for less-addictive opioids such as tramadol, known as Schedule IV drugs, that initial prescription can be followed by a six-month supply if the drug is deemed safe and necessary, Mach said.

At Angell, however, every refill of tramadol — whether the first or the 50th — is limited to a seven-day supply, Mach said.

"Each instance of the drug being dispensed must be reviewed and approved by a veterinarian," Mach said. "Because the more they have, the better to sell it, share it, abuse it."

In some cases, the medical needs of animals mean that pets are prescribed more doses of tramadol than humans would receive for pain management. Angell keeps fentanyl patches, as well as opioids such as buprenorphine and hydrocodone, locked in a safe.

Its veterinarians look for alternatives to treat animal pain after the initial seven-day supply is exhausted, hospital officials said. Often, that means transitioning pets to non-habit-forming pain medicines.

Dr. Lisa Moses, the head of Angell's pain and palliative care service, has added another preventative measure by requiring her clients to sign a contract when controlled substances are used to treat their pets.

Among the requirements, they agree that only Moses will refill prescriptions, that Moses will no longer prescribe the drug if another veterinarian gives them a refill, and that the client will not sell or share the drug.

Failure to abide by the agreement "may result in cessation of my pet's therapy and potential dismissal from treatment at this clinic," the document reads.

Susan Curtis, executive director of the Massachusetts Veterinary Medical Association, said the advocacy group is well aware of the potential for opioid abuse.

"We've been very proactive in educating veterinarians to educate pet owners, especially older pet owners, to be aware of who's coming in and out of your home and be aware of where you store the meds," she said.

The association partnered late last year with the Cummings School of Veterinary Medicine at Tufts University to create educational handouts for veterinarians across the state, including advice on how to dispense prescriptions safely and signs that might indicate they are being misused.

Mach, the Angell pharmacist, said new problems require a new approach.

"There's nothing spelled out about how to manage this," Mach said. "This is sad and it's scary, but it's something that we have to do."