Diet pills nowadays (at least the last time I looked) contain nothing but green tea extract and a LOT of caffeine (I guess you are supposed to jitter off the weight). Caffeine is usually bad for people with IBS. And you can get green tea from a tea bag and save yourself some money. The old drug they used to have in them that actually did suppress your appetite has long since been pulled from the market. Imagine that--it was dangerous. There is some evidence that green tea can supress the appetite, but either that's just a placebo effect or it wears off after long use, because I used to drink green tea all the time and I never really noticed a difference. But green tea does potentially have other good uses, so it's not bad to drink it daily. Maybe I just never drank my green tea strong enough to notice an affect.
My advice to you would be to not worry about the weight period. I just got finished reading an excellent book called "The Obesity Myth" by Paul Campos. He actually shows all of the studies that never get published in mainstream America because they prove that there is no link between health problems and being upwards of moderately obese (and he shows how horribly flawed and badly interpreted all of the other studies are). You have to be REALLY obese to acually have weight-caused health problems. Everything else can just be chocked up to lack of exercise. 30 minutes of brisk walking 5 days a week will make you healthy, regardless of weight. That's more important, really, than diet, although a healthy diet helps some too (especially if you have IBS). Not for weight loss, mind, but real health benefits. Weight loss, therefore, is all about conforming to an anerexoic ideal and not at all about health. Did you know that almost all models and actresses are underweight or nearly underweight according to the BMI? And studies have shown that that's more dangerous health-wise than being overweight. Take a good look at actresses and pin-up girls of the 40's, and then look at our ideals today. Our society is literally holding up anorexic people as an ideal look (Calista Flockhart comes to mind, even if she won't admit she's anorexic).
If you want to diet and potentially make yourself sick to lose a few pounds, you certainly won't be alone. But know that it's an established fact that 95%-97% of all diets fail. Within three years of dieting, at least 95% of all people will have gained back all of their weight and, more often than not, even more weight. Better to leave that weight alone. If you are above your natural weight (as determined by your genes), then it will likely come off of its own accord after the holidays when you go back to eating normally and doing your normal amounts of exercise. If you try and force it off, you'll likely cause yourself health problems and it'll just come back sometime in the future. And they've found that weight loss tied to excercise alone amounts to an average loss of 1-3 pounds over the course of several months. Exercise actually doesn't make you lose weight--probably because you replace fat with muscle, which weighs more. Exercising, therefore, can make you look better my making you more toned, even if you don't lose pounds. To say nothing for lowering your risk of hypertension, diabetes and heart problems.
I found this blog/forum that's really great: http://www.bigfatblog.com/ A good deal of the studies in "The Obesity Myth" are quoted on the blog, if you have the time to look for them. It's about fat acceptance, but there are people on there who are not fat, but who struggle to accept themselves as they are (i.e. they think they're fat when they are actually normal or even thin). I see some parallels between them and people with IBS, because they hear crap from doctors all of the time too, doctors that don't want to treat their problem (different reason, of course) and they often have fears of going out in public, being socially accepted, etc.
I am in no way implying that you are overweight, and I'm not trying to diss your wanting to diet--Lord knows I've done it. But this stuff makes a lot of sense to me (who has lost weight for several reasons, only to gravitate back to a size 14 and stay there for years), and it's important that everyone knows both sides of the coin. Summer before last I went from a 14 to a 10. I now wish I had never done it. Because I had been a size 14 for so long that I had come to accept that. When I started to lose weight, I became obessed with how thin I could get. I eventually stopped losing weight because I hit my plateau. Then I could only see how much I still wanted to lose. And when I inevitably started to gain some back, I started seeing myself as fat and had internal feelings of digust for myself, despite the fact that I'm still smaller than I was when I was a 14 and felt fine with myself. Dieting just made me really vain, killed my self-esteem and made me hate myself (heck, I got so bad I started sucking in my gut whenever I was naked around my fiance, despite the fact that he's never cared what I weigh and is quite overweight himself and I'm STILL smaller than I was when we started dating). Who needs that kind of crap? It's hard enough to have self-esteem when you have IBS-D. But to think you're fat on top of feeling dysfunctional is just too much. You'll just be unhappy all of the time.