There are pens specifically constructed to be used as self defense weapons; they are made of dense metal, they are solid and even have an end meant to be used to break car windows to get someone out who is trapped. A keen eye can see that these pens are actually weapons. One thing about the law is that if it is established that you carry an object intended to be used as a weapon then you could be found to be in the wrong even in a self defense situation.
However, if you just happen to be carrying around a well-constructed heavy pen that you happen to use on someone who attacked you, then you might not be so much in the wrong legally.
The Sanford Parker Ballpoint Pen is durable, rugged and sturdy, made of stainless steel. It definitely could double as a tactical pen for self-defense purposes, though would not be likely to be mistaken for a weapon.
The Fisher 400B Space Bullet Pen, made of brass and matte finish, was meant to be used by NASA to withstand high temperatures; it can be used under water and even in zero gravity. It’s nice and thick and would make a decent substitute for the tactical self-defense pen.
I have been training in martial arts since the early 1980s and have a 2nd degree Black Belt in Kenpo and have training in Eskrima. I believe everyone has a right to defend themselves by any effective means necessary.