Recommended by a friend who is a research scientist. This stuff helps me breathe. I've got COPD and am a Lung cancer survivor. I walk 3 to 10 miles each day. I bought these 600 milligram capsules thinking that I might get better results from taking smaller doses more frequently, but what I've discovered is that to really hit the therapeutic level, I need to take two of these pills at once, so i'll switch back to the 1000 mg pills after finishing this bottle. The product and the brand are both great. If you have difficulty breathing due to COPD, NAC is clinically proven to increase lung performance, and is definitely worth a try. There's also some evidence emerging that it helps with mental health and mood improvement