First you should be aware that cayenne comes in varying heat ratings called Scollville Heat Units or HU for short.
Most commercial regular store bought cayenne is pretty weak in the 30,000 HU range. Though Stanley does not specify all the well known herbalist prescribe 90,000HU or higher to have any therapeutic properties.
Many experienced MCers round here myself included do 1/4 tsp of the 90K stuff per drink and some do even more. Every now and then you get folks in here that say they use stuff in the 150K HU range (OUCH!) so it's more of a matter of individual tolerance.
Use the search function in the forum and see if you can find a blog that Zoe has on cayenne. Maybe someone will see this and post the link.