It is not uncommon to get a request for a trademark application for a good or service that is derived from cannabis. Cannabis is still a federally controlled substance under the Controlled Substance Act (CSA). Even if it is quite legal for you to purchase and sell marijuana for medical and/or recreational use in your city and state: it's still not legal to do so at the federal level.
Since federal trademark registrations are obtained with the United States Patent and Trademark Office (USPTO), a federal government agency, the USPTO will deny registration to any mark that violates the “Lawful Use Rule”, as stated in Trademark Act Sections 1 and 45. Because cannabis violates the CSA, it's an unlawful use and any trademarks that covers cannabis or byproducts of cannabis will be denied at this time. Certainly, over time, these laws may change, but for now it is not possible to attain a registered trademark at the federal level.
Although you cannot register a trademark for cannabis, you can register any ancillary goods or services that are not a product or byproduct of the “flower” with the USPTO. If your business sells more than just marijuana products, such as consulting services, helping people to find the right dosage and strands, educating others on cannabis use, brand apparel, then you can obtain a federal registration for those goods and services, but not for the actual products that stem from the marijuana plant itself.
Although federal trademark protections can not be secured, it is noted that "common law" rights may be enforceable. In this case, no registration is required, but you must be able to prove that you started using the mark in legal commerce before the potential infringer did. In addition, the Lawful Use Rule does not apply to common law cases.