RE: Milk Thistle
To find studies for it's effectiveness you would have to search medical journals and the like.
It does not however 'weaken' the oral taken but helps the liver combat the effects of toxicity that occurs due to the liver processing the substance.
In my opinion, it is a cheap supplement, it provides protection, why would you not use it during a cycle that will damage your liver?
I always use it during oral cycles and PCT.