There were 1,425 by the end of last month - compared to 1,251 last year.
Drugs seizures in jails have doubled since 2016, when the figure was only 715.
Former prison guard, John Cuffe, says many people throw drugs over prison walls into exercise yards:
"Even if we were spot them with the surveillance cameras on the outside, a guy would race up and fire them over the wall. You have 100 prisoners that exercise in any yard and maybe you could have as few as three staff supervising that. Once they grab that bag they rip it open and start throwing the stuff around amongst themselves. It's embarrassing. I know that in some places I put mesh and net over but their ingenius, it is quite easy to walk around these places and fire stuff over."
