Don't take my exact word for it, but I believe the efficacy of HGH depends on two main factors: growth stage and body's production of HGH.... it may work for you, but at 18 you are probably almost done growing, so it is kinda late to start, and it also depends on how deficient you already are at producing HGH. if your body is deficient, it will have a higher likelihood of having positive results from HGH