My son is 12 with ADHD Combined and ODD, he has been taking guanfacine ER since he was 9. We slowly increased his dose to 3 mg and he has been on that dose for a few years. We first noticed the drowsiness, as that went away we had to really tune into how his behavior changed or didn’t change. We found 3 mg to be the most effective dosage for him. I think it took a week or two for the drowsiness to subside where we could really see other changes.