INDIANAPOLIS (AP) — A recent trend of lacing mail with drugs has led to a ban on greeting cards in Indiana prisons.

The Indianapolis Star reports the April 1 change also bans colored paper of any kind and photocopies of white paper. Lined white paper in white envelopes and legal documents can still be sent to inmates

The ban comes after state prisons saw an increase in cards soaked in the liquid synthetic drug K2. The cards can be torn up and smoked, and can turn users aggressive and violent.

James Basinger is the deputy commissioner who oversees Indiana Department of Corrections operations. He says the ban was imposed because K2 is difficult to detect. Drug-sniffing dogs can’t catch it and drug tests aren’t effective.