'prenicious effects' = 'effects which can only be observed in long run, subtle/insidious effects'

Only a few days back I read some word starting with letter 'p', sounded something like prenicious (I understand there is no such word).

If used in a phrase it would mean something like: 'prenicious effects' = 'effects which can only be observed in long run, subtle/insidious effects'

Does this bring some word to your mind? Please let me know. Thanks.