Becoming less effective.
Ah, OK. Well I don't use it very much, but when I do, I haven't had any diminution of effectiveness. I find that it's best taken on an empty, or near empty, stomach (after a biggish meal I find that it takes a very long time to kick-in, and it's effectiveness is reduced, so I don't do that anymore. And anyway, sex on a full stomach is not that much fun for me).
I do get side effects though: mainly a headache, so I take a couple of Paracetamol at the same time, and my vision goes pink sometimes and bright whites appear rosy in colour. This passes quite quickly though.
So, so far I have had the effect I expect when I do take it.