In a criminal case, the judge determines the applicability of the law. (I am less familiar with civil cases so I won't address those.)
The jury, however, is the finder of fact and determines guilt. (Of course, a judge can always set aside a verdict.)
What you are saying above is that you want to judge on the applicability of the law (drugs should be legal). This is called "
juror nullification". It can be used for both good and bad. Basically, regardless of what the law says, under juror nullification you will find someone guilty or not based not just as to whether he violated the law, but whether the law is just.
A hypothetical example of a good use of juror nullification would be refusing to find someone guilty because they violated a Jim Crow era law.
During voir dire, you can say that you practice juror nullification. However, make sure you have done your homework and understand it cold. If you use this, make sure you believe it in your heart. Be prepared for the rest of the jury panel to be dismissed so you can be interrogated without prejudicing the other members of the panel.
If you feel strongly about juror nullification and the judge believes you are not using this as an excuse, you may indeed be let go. Both judges and lawyers generally hate the concept of juror nullification. They do not want the jury to have any sway on the fitness of the law. (I do personally believe however that the jury is the last measure of prevention against an unjust law.)
You may start your search on the topic here ->
http://www.fija.org/.
But once again I warn, don’t use this lightly. Be thoroughly prepared to explain why you support juror nullification.
Finally, if you decide that you really don’t believe in juror nullification, then pass, don’t use it as an excuse.