Calamus is actually the sweet root plant - it's a reed that grows in wetlands, and originated around India. There are a lot of different names for it, but it is the Acorus Calamus.
The oil is toxic in large doses, but small doses can be used therapeutically. In the holy oil of the OT, only the Levites could use it. Anyone else died if they used the holy oil - this leads me to believe that the Levites had something in their genetics that was receptive to the oil, in the same way that some oils are more toxic than others to people. There are a few oils that give me a very bad reaction, and I'm sure if I used those ones in large doses, they might kill me too - or at least make me very sick.
I started a study a long time ago on medicinal oils in the Bible, but do not recall most of what I learned. I only know that about calamus because when I was studying aromatherapy, I was going to make the holy oil for an assignment. Instead, I wrote a paper explaining why I can't make that oil (toxicity and my faith). I also wrote a paper on Frankincense and Myrrh. There are other oils in the Bible, too... iirc, there is one in Song of Solomon, in Psalms, and elsewhere, I just don't recall what they were or where to find them. But when I studied oils, it was all about the chemistry and the effects of the chemical makeup on the body. Unfortunately, back then, the Christian community was not open to different kinds of natural health, thinking it was all "new age" when in fact, much of it is "old age" ways of doing healthcare and preparing bodies for burial. Now it seems the Christian community is using essential oils a lot, but more for enjoying scented blends rather than for promoting health, which is what they should be used for. These chemicals, or oils, can be used through massage, adding it to lotion, cream or carrier oil, or they can be used in incense, as the nose membranes are very thin so they are absorbed quickly.