The 12 mg weekly limit applies to reta, not tirz. Tirz goes up to 15 mg weekly. Plenty of people go past that safely by raising the amount or injecting more often.
My subject is on 9 mg of tir split twice weekly. Progress stopped a while back, yet I want to avoid pushing the top end since 12 mg weekly is the ceiling I can reach. Has anyone tried adding reta alongside it? Any other ideas for what to combine with tir if reta does not work out?