According to US Defense Department regulation 5240.1-R, DoD intelligence components are allowed to gather information on US citizens in some circumstances relating to drugs: "C2.3.10. Narcotics. Information may be collected about a United States person who is reasonably believed to be engaged in international narcotics activities."
Source
Under Secretary of Defense for Policy, "Procedures Governing the Activities of DOD Intelligence Components That Affect United States Persons," (Washington, DC: United States Department of Defense, December 1982), p. 17.
http://www.au.af.mil/au/awc/a…
www.dtic.mil/whs/directives/c…
