I wouldn't compare lyrium to drugs like marihuana and other shitty substances which people use to "have fun"
Lyrium is closer to use of medical drugs for health reasons. Except it's not health reasons but a need to fight magical enemies
Even if Cassandra will be using lyrium, she won't do it to "relax", she will do it because that's what she has to do to counter demons and mages
I understand why some folk can be so much against it but... I can hardly believe that potential use of lyrium will handicap Cassandra in any way. She's a reasonable person, a tad angry but reasonable