As Daisy rightly points out, overtreatment causes weight gain but undertreatment can cause weight loss, early puberty (which can restrict final height potential) and polycystic ovaries, which reduce fertility.   I appreciate that it is difficult to get the balance right and I understand that most specialists believe it is better to slightly overtreat than undertreat as excess weight is a temporary problem that can be corrected, whereas losing height and putting fertility at risk are life-long problems.