I have tried all three of those, Megace, Marinol, and Periactin. Periactin did nothing for me though.
Megace I LOVE for the most part, it is a female hormone though, so I have often wondered how it would affect males... for me, it can cause me to be moody and I also miss my period that month (sorry if tmi, youre 18 right haha), and sometimes insomnia.
Megace/Megestrol make me soooooo hungry, I mean, I never feel full. In fact I had to start taking half the prescribed dose b/c I would eat until I was sick. I gain weight fast and easily while on it. Keep in mind it is only a layer of fat over everything, so the last time I used it, I also made sure to be "working" out, for me that was walking/jogging and stretching 3-5 days a week. This way, my body can use the fat to help turn it in to muscle, which is a more substantial weight.
When I am on megace I eat anything, even stuff I normally dislike, like italian food haha.
I tried Marinol b/c I had tried another round of Megace but was on the tablet form instead of the suspended liquid, and we (doctor and I) hadnt got the dose right so I had taken it a month w/ no effect. The marinol is the derivative of pot, hence the munchies. My ID doctor told me you should take I think 10-20 mg (I think again) before meals, and that you should "feel" it... so being high haha. Again, I took the wrong dose, too low, for about 2 weeks w/ nothing, so when I up'ed my dose to where I could feel it, I ended up getting sick both times and it never gave me the munchies, just the spins. Although, admittedly, the first say 15-30 minutes are hilarious <img src="i/expressions/face-icon-small-wink.gif" border="0"> but then I would just be sick for like 4 hours, and either puke or have to sleep (haha again).
So I encourage you to try both maybe. I know that on cysticlife.org, recently I saw a man had said he was on megace and I think was still on it.
Oh and another thing about megace, is that MY doctors told me to be on it a maximum of like a month or two, I think it can be bad for your bones as far as density, I know of a couple other CF'ers who are on it constantly and only up their dose when they start losing weight. For me, that seems unnecessary, especially if the fat is turned into muscle b/c it sticks on a lot longer. It is a catabolic steriod I think, that is why it makes you hungry kinda like predisone I have always thought.
And lastly, when on a consult in UCLA, the ID doctor there said now they use an actual anabolic steriod that starts with and R, I think a couple people have tried it on here. I kidna want to try that just so I can have 'roid rage' joking of course haha.
Just kidding, not lastly, I now have a feeding tube though, and I actually like it. I still might go on a couple megace kicks though to help get me bulked up and then the g-tube can help me maintain, but I lose weight quickly b/c my bugs take a lot out of me.