The US Dept of Justice Drug Enforcement Agency Diversion Control Division will sponsor the 12th National Prescription Drug Take Back Day on April 29, 2017. To find a location near you, see https://www.deadiversion.usdoj.gov/drug_disposal/takeback/ . As of today, many locations are listed in California, Oregon, Idaho, and Montana, but fewer in Alaska and Washington. However, they are adding collection locations daily, so check back later in April if you don’t find a convenient dropoff site.
Alternative methods of disposal are detailed at https://archive.epa.gov/region02/capp/web/pdf/ppcpflyer.pdf .