Poison pill - pre or post offer?

CFA Level II Corporate Finance
#1

The books and questions claim poison pill to be pre-offer defence that cannot be used after the offer is put on the table.

However everywhere in the market I read poison pill is made after the offer.

Example:

http://www.nasdaq.com/article/glaxosmithkline-amends-offer-in-response-to-human-genome-poison-pill-20120523-01167

So where is the truth?

#2

For the exam, go with the CFA book.

#3

I don’t think they mean that you cannot use it post-offers, only that it is more effecive as a pre-offer strategy to deter hostile takeovers.

#4

poison pills are implemented in advance of a takeover. I guess they are actioned once a takeover occurs (e.g. with dilution) but they are a pre-takeover strategy