In practical terms, none in the UK. Over the years restrictions have been brought in to remove potentially hazardous meds which could be used to more easily access CTB.
Exception could be Paracetomol, an overdose of which can lead to painful, protracted and undesireable organ failure. If intercepted in time you may just finish up on lifetime dialysis or a transplant waiting list. I only mention this as if I don't it's almost inevitable that someone will bring it up.