My daughter was on periactin twice. The first time it made her very drowsy. The doctor said that the drowsiness would go away after a few days and he was right. It worked well to increase her appetite too - she gained 2 pounds the first week.
The next time it didn't work at all. It didn't make her drowsy. It didn't stimulate her appetite. It was just one more pill to take.
We tried Resource and the Boost chocolate drinks but to no avail. She just doesn't like them.
Now I just make meals that I know she will like. It is contrary to my philosophy that kids should eat/try what the rest of us eat, but the consequences for her are too scary.
Best of luck,
mom of two - daughter, age 8, DD508, and a son, age 9, no CF