I'll assume that you are talking about the U.S. government here...
Sending letters, email, phone-calls etc. to government agencies won't accomplish anything, I assure you. This government is completely out of the citizens control nowadays. All of our 'representatives' (at least, the ones in influential positions) are controlled by powerful interests. I'm involved in left-wing politics, and I have found that it is nearly impossible to convince the government to go against those groups. You can't even persuade them to respect basic civil rights and the constitution. That is something that is very popular with citizens, it shouldn't be hard to convince them to do this, but it is. I have called them hundreds of times, sent dozens of letters and emails, signed numerous petitions... it makes no difference to them. They are paid lots of money to ignore me.
That is why your calls will do nothing. There are many groups and industries that are opposed to drug legalization, because they make lots of money from it being illegal. They have a lot more influence with the government than you have. A call from a constituent means nothing to them, because they have access to lobbyists that will give them ANYTHING to influence their opinions, from dinners at expensive restaurants and vacations in the Bahamas, to tens of thousands of dollars in cash, to hookers and coke. On top of that, drugs are a 'third rail' type issue and not nearly as popular as protecting civil rights.
Calling them works occasionally, it worked last year when conservatives stopped the immigration reform bill. But they had the media on their side. Drug users aren't so lucky. The media gets lots of advertising money from big pharmaceutical companies and other industries opposed to drug legalization, and they don't like to make their advertisers angry.
I think it would be possible to convince them to stop persecuting drug users, but it definitely won't be as easy as calling your senators and the DEA a few times. There are serious problems with the political system that must be fixed first. The main problem is:
"I see in the near future a crisis approaching that unnerves me and causes me to tremble for the safety of my country...corporations have been enthroned and an era of corruption in high places will follow, and the money of the country will endeavor to prolong its reign by working upon the prejudices of the people until all wealth is aggregated in a few hands and the Republic is destroyed." - Abraham Lincoln
^^ This is happening, or it happened already. Americans have got to put a stop to it. Until that is done, I doubt drugs will be legalized. Too much money involved...