As a general rule you shouldnt be touching steroids whether otc or not until you are at least 25 in my book and then the subject should be throughly researched in terms of sides long and short term .You should also have supplements in place to cover post cycle therapy and also whilest on cycle, if you do decide to go ahead.
At 18 you have no need to mess with your endocrine system (hormones) Your testosterone levels will be reaching a peak and your estrogen level will be low.Utilise this by training hard, sleeping well and eating regular and clean. Steroids only work if all the other factors are in place anyway.
It also no good taking something like this if you are constantly worrying about what it could be doing to you.
My suggestion would be to stop taking them -and go for basic supplements such as creatine.(tame i know )
At 18 you have as i said earlier the raw material to grow-testosterone . it will never be as high again as in your late teens and early 20s .