Yes, in order for the federal government to force the states to allow women and 18-20 year olds to vote, an Amendment was passed. And in order to repeal the (nationwide) Prohibition Amendment, another Amendment was passed.
That does not mean that to legalize drugs at the federal level an Amendment would have to be passed. Even under a strictly federal system (which I would favor, but which has been sadly leacking since FDR's presidency) an Amendment legalizing drugs would only be needed if the federal government wanted to not only legalize drugs at the federal level, but also make it unconstitutional for the states to keep drugs illegal. I'd leave drug laws -- like virtually all the criminal code -- up to the states. So I do not need to advocate a constitutional Amendment.
If marijuana (or any other drug) is legalized, I'd prefer that it come about by a change in the law rather than a constitutional amendment.
That said, I believe a constitutional amendment could accomplish the same thing. Keep in mind that an amendment to the federal constitution would only apply to the federal government, and could be written to take away the power to regulate/restrict drugs. Only if the individual state adopted the amendment would it apply.
Case in point: California did not adopt the second amendment -- there is nothing in the State constitution regarding the RKBA. California can make all guns illegal tomorrow.