Hi, E13mom, Welcome.
What happened with our Vets is multifactorial. They not only had Mestinon - which not all of them did - but had super vaccines, pesticide exposure and so many other variables, not to mention the heat factor of Iraq. PTSD in most cases is directly related to the trauma of a "war" situation. The unexpected nature of what they went through is unsettling to the nervous system as well.
When a body has too little or too much acetylcholine (ah-seat-ill-co-lean), it can make someone "feel" depressed. When the body gets weak, it can honestly make your mood lower. It "might" have something to do with seratonin, however, if the dose was small there would have to be some kind of underlying problem which it enhanced. It isn't known to have that as a primary effect. See this basic site and the PDF below.
http://www.fpnotebook.com/Neuro/Pharm/Mstn.htm
Is your son on any other medications or supplements? Does he have any other medical conditions?
Mestinon can cross the blood-brain barrier but five days of it should not cause any serious effects or long-term effects. How much was he given? If he was given a huge dose, that would not only be hazardous since he doesn't have MG but truly STUPID on the part of the doctor. But the effects would be muscle weakness, drooling/increased saliva, bradycardia, shortness of breath, etc.
As far as the panic attack, when did it occur? How soon into taking the Mestinon? Was he in a hospital setting? Could he have been worried about taking any drug and might any drug have that set that into motion? When I had my first strep antibiotic shot, it scared me; mainly because the doctor was a veritable robot.
What made the doctors reach for the Mestinon in the first place? There has to be some kind of underlying medical condition for them to go that route. Do you wish to share?
I think you should take a deep breath and realize that this was a temporary situation with Mestinon. No, it's not fun to have a child have a reaction to any drug but the drug is out of his system now. It leaves the body in hours and its effects can last up to 2 weeks on the immune system (not the CNS system, which includes seratonin).
If there is some kind of metabolic issue, maybe a trip to an endocrinologist might help. The other thing Mestinon does is to increase peristalsis, the way food goes through the GI tract. It makes things move through more quickly. Perhaps there was an imbalance of electrolytes and fluids too. Did they check those? Those changes alone can look like someone is having a panic attack. Low sodium, for example, can cause extreme agitation. And having anxiety and having a panic attack are 2 different things! So if the doctor wasn't well versed in psychiatry, they may not have done a thorough enough evaluation to decide which one your son was having.
I've been on Mestinon for over 11 years and have had absolutely no ill side effects. I have also had panic attacks (NOT anxiety) from some fairly traumatic medical situations. Just wanted to add that, so you know that I have experience to back up what I'm saying.
If we can help in any other way, please let us know.
Annie