After making no progress in the gym and having libido problems, I had a blood test taken for Testosterone levels and tested out at 160 (normal levels are between 300 and 1200). My doctor gave me a book written by a physician who noted that the most common cause of low T levels in men is a zinc deficiency. I started taking GNC Zinc and, unfortunately, would throw up less than three minutes after I'd take a pill. With the Nature's Plus timed release, I don't even get an upset stomach. After three months my libido problem disappeared and I'm now benching upwards of 250 (I weigh 165). Highly recommended!