Law enforcement officials in Ohio report an increase in the amount of Suboxone, a drug used to treat opioid addiction, being smuggled into prisons, often through the mail.

In one case, a greeting card mailed to an inmate contained 20 strips of Suboxone, The Plain Dealer reports. The increase in Suboxone smuggling corresponds with an increase in heroin and opioid use in Ohio, the article notes.

In the past several years, prison officials throughout the country have reported that Suboxone is being smuggled in through ingenious means, including crushing the pills into a paste and spreading it over children’s artwork or under stamps. Smugglers are also taking advantage of the formulation of the drug that comes in thin strips, which can be hidden behind stamps and envelope seams.

“While I cannot provide specific numbers, I can say that we have seen an increase in attempts to convey Suboxone in our facilities,” JoEllen Smith, a spokeswoman for the state, told the newspaper.

Prison officials have notified the Ohio State Highway Patrol more than two dozen times in the past two years about Suboxone smuggling.

According to Orman Hall, Director of Ohio Governor John Kasich’s Cabinet Opiate Action Team, many people using Suboxone illegally are trying to self-treat their opioid addiction. He noted research shows that when it is combined with traditional forms of counseling, Suboxone is a safe and effective treatment.