As an 18-year-old, finasteride 1mg is generally safe for treating hair loss, but since your body is still transitioning out of adolescence, it is best to use it under a doctor's supervision. Regarding your concern, sexual side effects, such as a decreased sex drive or difficulty maintaining an erection, can occur, but they are uncommon, affecting only a small percentage of users. If they do occur, they are typically mild and usually resolve completely after stopping the medication. There is no need to panic. Simply monitor how you feel and stay in touch with your doctor to ensure this remains the right treatment for you.