Information I have read on Milkthistle and NAC show its best used while the liver is being exposed to toxins(ie Acetominifen, M1T) by aiding the liver and its metabolism to breakdown toxins while lessening the damage those toxins do to the liver.
I believe it was Big Cat who mentioned that increasing the liver's ability to break down toxins might hinder gains while on cycle. This is pure hypothesis and to me it seems like myself and numerous others get and keep the same gains as when we did not use MT and NAC.
IMO it is foolish to hope that MT/NAC/RALA will REPAIR damage brought on by M1T when there is little evidence showing that this is what MT/NAC are for. Evidence points to them being protectants not magical liver cell replacement pills.
As an aside, the level to which NAC/MT can actually protect the liver from harsh methyls like M1T is not fully understood. We've seen very high liver values even from individuals who used them on cycle...so be cautious.
Also, research has pointed out that inconsistent use of MT/NAC is actually WORSE than either not taking it all or taking it consistently everyday. In other words, while on cycle DO NOT forget to dose your MT/NAC.