Kind of strange. Decapeptyl is triptorelin a GnRH. It will cause a flare in testosterone to start with but T levels should be down to surgical castrate levels in two to four weeks. So the only thing I can think of as a "test" is he wants to put you on it for a specified time.

Hi, yes I forgot to mention:

It is a one time injection and he wants to take blood samples after 24 hrs - is this a usual procedure?

Joe