Acorus calamus is one impressive plant when you consider that it has been used as a natural medicine for thousands of years over vast geographical distances  and that modern science has identified at least 10 uses for Acorus calamus (sweet flag)!
The Elephant In The Room: Is Acorus Calamus Too Dangerous To Use?
There's no shortage of studies conducted by proponents of Ayurvedic medicine that proclaim sweet flag to be a wonderful plant without mentioning its dangers, and the same certainly holds true for alternative medicine websites written by laypeople. If you only look at those, you might quickly conclude that Acorus calamus won't do you any harm even if it doesn't end up improving your arthritis symptoms. There is, however, an elephant in the room, and one we think we ought to address before we even look at whether sweet flag can help you out with your arthritis.
Carcinogenic in larger quantities because of its active component beta-asarone , sweet flag has been banned by the FDA in the US . The European Medicines Agency, meanwhile, takes a more optimistic approach, saying:
In view of the toxicity of α- and β-asarone, their concentration in herbal medicinal products should be reduced to minimum and diploid varieties should always be preferred. In analogy with the food regulation (limitation of the intake of β-asarone from food and alcoholic beverages), a limit of exposure from herbal medicinal products of approximately 115 µg/day, i.e. about 2 µg/kg bw/day could be accepted temporarily until a full benefit/risk assessment has been carried out. 
If you do use Acorus calamus, in other words, do so sparingly. This is something even the most fervent believers in Ayurvedic medicine agree with, as far as I can conclude — everything I've looked at advocates not using more than a gram of sweet flag — called "Vacha" in Ayurveda speak — a day and not continuing use for longer than four weeks. The plant, they say, is simply too strong to use more for longer.
Does the method of use make any difference, though? Were you to use sweet flag as a treatment for diarrhea, stomach ulcers, depression, or epilepsy (yes, all things Acorus calamus is said to help manage), you would most likely look at ingesting sweet flag's rhizome extract, powder, or essential oil.
While little information is available on the topic of the potential carcinogenic effect of using beta-asarone externally, we should at least conclude that sweet flag's hallucinogenic properties will not affect you.
What Is The Evidence That Acorus Calamus Can Help Relieve Arthritis?
Acorus calamus acts as an analgesic — offering you pain relief — and as an anti-inflammatory.  Together with reports that sweet flag is "known as an old folk remedy for arthritis" , this is pretty much what the evidence boils down to. As for the latter, I'm not sure whether to dismiss it as an "appeal to antiquity" fallacy or to assume there is some common sense involved here — if the ancients used sweet flag to relieve their arthritis and it worked for them, well, that may mean it will work for you, too. Which still doesn't mean it's not carcinogenic, of course, and that's one danger that is hard to just ignore.
The choice is not, of course, between relieving your arthritis with sweet flag and simply accepting your symptoms. There are many other arthritis treatments around, after all, ranging from natural and also dangerous (Comfrey ), to natural and safe (Turmeric ), and things your allopathic doctor may suggest, like corticosteroid joint injections, physical therapy, and surgery.
In general, unfortunately, there's just not enough evidence that any form of alternative medicine helps relieve your arthritis , while modern medicine really does quite well at managing it. In the face of the concerns associated with sweet flag, we'd not advise anyone with ready access to twenty-first century medicine to experiment with using it.