Welcome to the forum, Hanco~
Is this not something your prescribing doctor is willing to help you with?
I don't have any personal experience to share with you, but perhaps someone else who does will post for you.
If you want to use the search icon at the top of this forum page and type in "talwin nx", that will help you find out if there are some older posts where this has been discussed. Here is one I ran across that may give you a little feedback:
http://www.medhelp.org/posts/Addiction-Substance-Abuse/talwin-nx/show/1119501