i must admit i used synthol back about a year ago... It was a big mistake in my opinion. The shit i had hurt like a mother fucker. The pain stayed and stayed, i never got used to it. I couldn't fullt extend my arms, not just from the pain, but i couldn't.
It works well though, but only for a short period of time. At that time my bi's were like 16" and they inflated to 18" for the 3 months that i used it. After i stopped it went away after a month or so, but by that time i got bigger. My arms were like 16.5"...
Then i just decided to bomb my bi's and over an 8 month period through natural gains and juice they got back up to 18"
I would stay away from it though, its good for the time if you can stand the pain. But over the long run it was a waste of $300