The U.S Food and Drug Administration is criticizing a letter sent by U.S. Sen. Angus King, I-Maine, and seven other senators over the agency’s policy on prescribing OxyContin to pediatric patients.

King joined the other senators – including Sen. Joe Manchin, D-W.Va., Sen. Ed Markey, D-Mass. and Sens. Kelly Ayotte, R-N.H. and Jeanne Shaheen, D-N.H. – in a Sept. 9 letter to lambaste the agency for what they say are rules that would potentially increase the availability of opiates for children ages 11-16 who are prescribed the drugs to control pain.

Public health advocates say that prescription opiates are often a gateway to heroin use. As prescription opiates have expanded in use since the 1990s, some patients – about 1 in every 15 who receive an opiate prescription – turn to heroin. The prescription connection has been considered a major factor in the exponential increase in heroin use since then. In Maine, overdose deaths and heroin usage have soared during the past three years.

“An increase in the availability of opioids like OxyContin to children – and the potential for abuse – poses a serious U.S. public-health issue,” the senators’ letter said.

The FDA approved guidelines for pediatric use this summer, but federal officials point out that doctors were already permitted to prescribe OxyContin to pediatric patients.

Eric Pahon, an FDA spokesman, said in a statement that the new rule does not expand access to OxyContin, but sets prescribing standards.

“It’s important to stress that this approval was not intended to expand or otherwise change the pattern of use of extended-release opioids in pediatric patients,” Pahon said in a statement. “Doctors were already prescribing it to children, without the safety and efficacy data in hand with regard to the pediatric population.”

Pahon said that “since the drug was being used to treat children, we wanted prescribers to have the information they needed.”

King, reached last week by phone, said he was “puzzled” by the FDA’s response, saying the agency should have had an advisory committee on the topic, to hash out the potential impacts of the new guidelines on children.

“We should have heard from the FDA the plan so we could have understood the consequences of this decision,” King said.

Scott Ogden, King’s spokesman, said while it’s true doctors could have prescribed OxyContin to minors as an “off-label” prescription, many doctors would be reluctant to do so.

“To prescribe a drug off-label means to prescribe it for something other than what it was originally approved for by the FDA,” Ogden said in a statement.

Ogden also pointed out that once a drug receives an “on-label” designation by the FDA, it’s legal for pharmaceutical companies to market and promote its use.

“The FDA’s actions give the producer of OxyContin … the legal right to begin advertising to pediatricians about the use of OxyContin to treat 11-16 year olds under certain circumstances. We have seen the devastating impact of this targeted advertising as the number of opioid prescriptions quadrupled between 1999 and 2012,” said the letter signed by King and the other senators.

The FDA guidelines say pediatric patients ages 11-16 should only be prescribed OxyContin if they require around-the-clock care and no other opioids or painkillers had been effective in alleviating pain.

Pahon pointed to FDA literature on the decision that points to how the pediatric patients needing OxyContin are those in severe pain with often debilitating diseases.

“These patients are typically being treated for cancer or recovering from extensive procedures, such as spinal surgery, such that they are expected to require several weeks or more of pain management with an opioid,” according to the FDA website.