Dozens of hikers became ill

It also appears that, while activated charcoal is used in water filters to, yes, get the viruses out, the studies on using internally are few and mostly not conclusive, with some suggesting it might help, others not. I noticed that a number of sources point out that common side effects of consuming it include vomiting and diarrhea, though constipation is also mentioned, as well as the risk of it absorbing other meds, as Sarbar noted.

Overall, I’d say it’s a big maybe, and for me, that doesn’t get it a place in my FAK. Note too that it isn’t a systemic thing—it can only absorb what’s in the stomach and (maybe?) on through the digestive tract. 

rebeccad, I'm sure you already know this but charcoal is the "go to" for emergency poisons in emergency rooms because it works so well. As you pointed there cases where it doesn't have much affect but it will not hurt you. As a user of charcoal for many many years I can personally testify that it is a great product. It works wonders on insects bites. I mostly use it for indigestion, heart burn, and diarrhea, where I've had 100% success. We keep a rather large supply of it in our house. The only side-affect I've ever had was a day's worth of constipation.

The only risk I know of, like already mentioned, is its use with meds. This alone should testify to how well it works.

In my FAK is about 20 to 30 tablets (purposely not capsules) that I bet if I weight them would be less than an ounce.

This is just one study there are many many more. With this I'll leave this topic alone as it could become my "soap box"

www.ncbi.nlm.nih.gov/pmc/articles/PMC1306980/

Poisoned patients were first treated with charcoal more than 150 years ago. Despite its almost universal acceptance today, activated charcoal's role has been overshadowed by the emphasis on treating poisoned patients first with gastric emptying. We review the current use of activated charcoal and recent studies that suggest that activated charcoal may be the single most effective treatment in many types of poisoning. New explanations for the mechanisms of action include “back diffusion” and disruption of enterohepatic loops. Clinical data endorse a new and aggressive role for activated charcoal in the management of poisoned and overdosed patients.