My son was on Adderall from 2nd grade until 5th Grade and switched to Focalin XR in 6th grade. He is now 18 years old and has been on a steady growth pattern. He only takes the medication on school days. Not weekends, holidays or summers. He is not 6 ft tall, but neither is my husband. I am only 5ft tall, so I couldn't expect him to be tall. You have to look at your family and your child's growth pattern to determine what is normal for him. Your doctor should be monitoring that. Also, how old is your son? If he is a late birthday, that would explain him being behind the others. My son had a huge growth spurt the summer after 6th grade. His birthday is in April. It was always a tough adjustment the first few weeks of summer when he was off his meds, but things got better. Focalin and other similar medicines are not cumulative. You take it, it works, it wears off. It is not necessary to take it every day for it to work. My son would also not take his medicine on Field trip days. Some of the best advice I got, was to not force the meds. As he is maturing, allow him to make some choices (on a day that would not be critical.) In 4th Grade, after all the TAKS tests, I gave him the option to try without medicine. He went three days and then came crying to me that he needed his medicine. It has been his decision ever since. Now his last year of high school, he only takes Focalin when he has a test.