Trump spokeswoman Sarah Huckabee Sanders said the formal declaration was delayed because of the "in-depth legal process" required to do so.
Trump acknowledged the effort to combat the opioid crisis will likely take years or decades but said, "We are going to overcome addiction in America".
"I have spoken to the president in depth about this epidemic and the devastating impact it is having on our communities in OH and around the country, and I know he is committed to addressing it in a comprehensive way", said a statement from Portman, who urged Congress to provide more resources to fight the epidemic.
"That's what I think is so important".
The declaration lasts for 90 days and can be renewed, but it comes with no dedicated funds.
The president instead will ask the acting secretary of Health and Human Services to declare a public health emergency, which allows the agency to waive restrictions and deploy medical personnel to rural areas where medical options are limited.
Trump said the Federal Drug Administration is requiring drug companies to provide a more training to prescribers and he promised to prosecute "bad actors" who illegally prescribe opiates.
The Public Health Emergency Fund at HHS now stands at $57,000, according to an agency spokesperson, and officials said the president's declaration won't yet include a request for Congress to replenish the fund. Maggie Hassan, D-N.H., unveiled legislation which would invest $45 billion to combat the opioid crisis and called on Trump to direct the government to negotiate lower prices of naloxone, an opioid overdose reversal drug used by first responders. Michigan Attorney General Bill Schuette was at the White House for the presidential announcement.
"In Wrentham, signs with "#2069" on them show how many people died a year ago in MA from overdoses.
Trump said the Department of Homeland Security and United States Postal Service have enhanced their investigation methods for imports of fentanyl, which is 50 times more deadly than heroin. "He would say it over and over again", Trump said. "We need to be in it for the long haul". "Part of the reason we need the wall is for drugs".