In December 2014, the Washington Supreme Court added a new comment  to Washington RPC 1.2 in order to give some comfort to Washington lawyers who want to assist clients who wish to engage in legal marijuana projects. The comment, in its entirety, states:
The court’s new comment does not go as far as some advocated it should. The King County Bar, for example, proposed a much more specific rule. And the WSBA Committee for Professional Ethics (CPE), asked to comment on the King County proposal, also advocated for a more specific and robust rule. But this is what we got. As a consequence, the CPE issued a much more far reaching opinion in 2015. The opinion can be found behind the following link: WSBA Advisory Opinion 201501 . Here are the questions the opinion addresses and the short answer:
1. May Lawyer A advise Client A about the interpretation of and compliance with I-502 and the CPPA without violating the Washington Rules of Professional Conduct (the “RPCs”)? Short Answer: Yes, qualified.
2. May Lawyer B provide legal advice and assistance to Client B in the formation and operation of a business entity so as to comply with I-502 and the CPPA without violating the RPCs? Answer: Yes, qualified.
3. May Lawyer C own and operate an independent business in compliance with I-502 and the CPPA without violating the RPCs? Answer: Yes, qualified.
4. Assuming that Lawyer D’s need for and consumption of medical or retail marijuana do not otherwise affect Lawyer D’s substantive competence or fitness to practice as a lawyer, may Lawyer D purchase and consume marijuana in compliance with I-502 and the CPPA without violating the RPCs? Answer: Yes, qualified.
5. May Lawyer E engage in the implementation of I-502 the CPPA and, if Lawyer E’s competence and fitness to practice as a lawyer is not affected, purchase marijuana subject to I-502 and the CPPA without violating the RPCs? Answer: Yes, qualified.
As you might suppose, the CPE’s opinion goes into considerable detail about the history of the issue and its reasoning. Footnote 17 to the opinion, for example, also provides references to what other states have done about this issue. Anyone whose practice (or interest) is impacted by this issue should read the opinion.
Update (November 9 2020):
In 2018, the Court amended the relevant comments to the Rules of Professional Conduct to read as follows:
RPC 1.2, comment 
“Under Paragraph (d), a lawyer may counsel a client regarding Washington’s marijuana laws
and may assist a client in conduct that the lawyer reasonably believes is permitted by those laws. If Washington law conflicts with federal or tribal law, the lawyer shall also advise the client regarding the related federal or tribal law and policy.”
“[Comment  adopted effective December 9, 2014; Amended effective September 25, 2018.]”
RPC 8.4, comment 
“A lawyer who counsels a client regarding Washington’s marijuana laws or assists a client in
conduct that the lawyer reasonably believes is permitted by those laws does not thereby violate RPC 8.4. See also RPC 1.2 Washington Comment .”
“[Comment 8 adopted effective September 25, 2018.]”
Since the Court revised these marijuana-related comments, the WSBA Advisory Opinion has not been revised or withdrawn. Nonetheless, some of its conclusions are even more problematic than they were before the Rule comments were revised. The problem arises principally because the new comment  added to RPC 8.4 fails to address, entirely, whether a Washington lawyer may, without putting himself/herself in ethical jeopardy, actively engage in a marijuana business personally or personally purchase and/or consume marijuana even though it violates federal law. The Advisory Opinion conclusions (3)-(5) give a “qualified” answer to the questions raised there and state that a lawyer may personally do so (quite apart from assisting a client), so long as the lawyer’s competence and fitness to practice are not affected. The Court’s silence on the question of personal use, purchase and consumption in 2018, or since, is deafening.