I want to second Lisa's recommendation about distracting yourself and calming down. For me, it's become standard proceedure to watch an engaging movie or TV show if I think I might be reaction to something but am not sure, or if I've just taken something new and I want to gauge my reaction to it. My reason for doing this is to completely take my mind off it so that if a symptom is bad enough to cause me to recognize it I can be certain that it wasn't brought on by a hyperattentive emotional state.
Having severe MCS and knowing what tricks our emotions can play on us, I find that in order to properly manage my CFS I have to "outwit" my emotions so to speak on order to get an accurate read on what I'm reacting to. Taking a hot shower, as Lisa suggested, for me is like hitting the "reset" button on both chemical reactions and hyperattentive emotions, and has been a very helpful tool in my arsenal. I also try to test things multiple times, usually with several days to a week inbetween testings, to make sure it's accurate. The same symptom every time is accurate...inconsistent symptoms means it's something else. The second time of testing it will likely be more accurate than the first because you can be more emotionally prepared for what to expect. The key is to distract, distract, distract....if a symptom forces itself upon you, you'll know it. If it doesn't, or if you're not sure, you still got an enjoyable activity out of the deal.
If you're concerned that taking the pill is going to potentially make you pass out, stop breathing, or any other symptom that would require medical attention, then a good distraction tool is to spend the first hour or two of taking it talking with a friend either over hte phone or in person (online won't work for this). Let them know that if you suddenly stop talkign or appear paniced that they should call 911, but that probably won't happen. Then just have an enjoyable, goofy, long conversation. You can relax because you'll know if something happens you'll be taken care of, and it'll take your mind off it in the meantime.
As Lisa said, make sure you start with as small an amount as possible. For me, even after having severe, life-threatening MCS for several years, the only time I've had severe reactions to medications has been either from taking a full dose of something bad, or from taking a smaller dose of something that I had previously reacted severely to. In my experience, a miniscule dose has not been the thing to cause severe problems; it's generally when I think that since a tiny dose is fine than more should be no problem that I run into trouble.
Also, again speaking as someone who has severe MCS, some days I'm just too emotionally involved to try anything or accurately test anything or evaluate reactions. My mind is racing on how I'm going to manage my life without ______ if indeed I do react or am reacting, I'm frustrated about the situation and I'm generally sleep-deprived enough for it to affect my clear-headedness. When that happens I just take the day or two or even the week off, because I just cannot afford to have my emotions influence whether or not I get well. Reactions are too severe and make me too ill to be wasted on times when my emotions might influence the results.
So I just wanted to encourage you...I know it's scary to be assulted with all these reactions, but with dilligence, hard work, and time it
is possible to learn how to not let those emotions get in the way of your seeking treatment and testing MCS reactions.