Wikipedia on binomial nomenclature:

In modern usage, in writing the first letter of the first part of the name, the genus, is always capitalized, while that of the second part is not [...] Both parts are italicized. Thus the binomial name of the annual phlox is now written as Phlox drummondii.

Also:

The binomial name should generally be written in full. The exception to this is when several species from the same genus are being listed or discussed in the same paper or report, or the same species is mentioned repeatedly [...]

(In my opinion "cannabis" in most cases seems like a common name rather than a taxonomical name, and it looks weird capitalized.)