Under current policies, people who have used drugs earlier in life cannot be hired by the FBI, even though there are people at other agencies who have used drugs before.
The main reason for this potential change in policy is that many managers have been frustrated with not being able to hire people who have used marijuana in college.
The CIA and State Department currently allow prior drug use. The rules, however, would not allow these people to have roles as the famous "G-Men" who actually conduct most of the criminal and terrorism research. Source