I used it for my last two half marathons. I can say that it definitely worked, I finished both marathons as if I had just had a walk around the park. I was well trained, but I could still feel the difference in endurance when taking the ketone ester. I was a bit nervous the first time I took it, I run around the house just in case I had any adverse effects. However, I had no adverse reactions whatsoever. It does taste a bit sour the first time, but I quickly got over it. I tried taking a little bit before going to work for a few days and I can say it did provide extra mental clarity.If this product was cheaper, I would be a loyal customer, but at $33 per bottle I can only afford it a few times a year. So far I bought two packs, one for each half marathon. Well worth it.