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Both of these references outline the data that state HCG in high doses can be toxic to leydig cells, but I'm going ot have to rediscover my references for the data that shows 300 is about the limit of what they can tolerate. In your favor though, nolva is proven to protect leydig cells from this, at least at 'reasonable levels.' I always take nolva with hcg.
The data I've recovered after a short search does in fact say that 500iu is the highest dose well tolerated. however that same data suggests that 500iu gives no more benefit than 350iu is most subjects. Additionally, since HCG seems to have an active period of between a day and two days, injecting every day is unnecessary. Since the largest effective dose (350iu) is not always necessary, and levels about 500iu can cause damage, I am much more likely to stick with 250iu for safety and economy.
As for the use of HCG as PCT, I stand by that too. Back when it was suggested that HCG was too suppressive to be used during PCT the protocol was 5000iu per injection, or more. Of course it was suppressive. Using the minidose schedule, in conjunction with nolva, and especially with the addition of clomid, there is little to no estrogen rebound, no damage to natty test, and in my experience a soft landing.