I’ve clamped and clamped and clamped some more, and the only thing that I’ve had problems with is slight desensitization (when doing it for ridiculous amounts of time, and long sessions at a time), and the classic clamp discoloration many clampers have had to deal with.
Other than that, I’m fine. The main thing is that you don’t do it when you’re a newb. You need to have your dick conditioned for at least a year (and when I say a year, I mean a year of near DAILY jelqing, kegels, etc). That and don’t want too much too fast. That was my problem, I started out clamping and within a month, I was clamping 3-4 times a day, 20 minute sessions each. Do NOT do this. Start out with one session per day, no longer than 5 minutes, and keep it there until you’ve got that down, then gradually step up your routine very slowly and methodically.
And btw, if you notice ANY discoloration, STOP IMMEDIATELY. While the discoloration itself isn’t particularly harmful to the functionability of your penis, the look of darkened color against lighter skin can freak out girls, especially when they go to give you a blowjob. Then you have to make up some bullshit story about how your dick got crushed this one time and that’s how it got that way, and then they’ll feel so sorry for what you went through and fuck your brains out all night long because of it and… Hm. Ok so maybe that wasn’t the best example to deter from over-clamping. My bad. =P
edit: It helps being huge if you’re discolored. If you’ve clamped and you’re still little, hang yourself. rofl j/k
edit’t: No, clamping doesn’t cause closing of the urethra. That’s most likely caused by another pre-existing medical condition, or poor diet. If you drink tons of water, I doubt you’ll have problems with your urethra TBH. Clamping has made my urethra fucking huge (it’s like Niagra falls when I go weewee now). =p