November 2018

Marijuana, derived from the cannabis plant, is a drug with recreational and medicinal uses. Its use, however, can have implications for highway safety. Reviews of experimental studies report that recent marijuana use can reduce performance in both simulated and on-road driving, but the effects of marijuana are inconsistent. In some studies, drivers who smoked marijuana had slower reaction times and greater lane position variation, compared with drivers in placebo conditions. Other studies failed to find such differences in reaction time and lane position variation. In addition, drivers who recently smoked marijuana drove more slowly and allowed more headway when following other vehicles, compared with drivers in placebo conditions.

Driving under the influence of marijuana is illegal in all 50 states. Marijuana is also classified as a Schedule 1 controlled substance and is illegal under federal law. However, certain types of marijuana use have been legal under state law in some parts of the country since the 1990s.

Twenty-two states allow medical use of marijuana, while another 14 states permit the use of specific cannabis products for designated medical conditions. Ten states and the District of Columbia permit both medical use of marijuana and recreational use for adults 21 and older.

State Type of law Age restrictions Possession limits Retail recreational sales allowed
Alabama
limited medical; effective July 1, 2014 none specified cannabis product that contains no more than 3 percent THC
Alaska
medical; effective June 1, 1999 none no more than 1 ounce
recreational; effective February 24, 2015 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce October 2016
Arizona
medical none no more than 2.5 ounces
Arkansas
medical; effective November 9, 2016 if under age 21, cannot smoke medical marijuana no more than 2.5 ounces
California
medical; effective November 6, 1996 none no more than 8 ounces unless a greater amount is deemed medically necessary
recreational; effective November 9, 2016 if under age 21, not allowed to purchase, possess, or use no more than 28.5 grams of cannabis not in the form of concentrated cannabis and no more than 8 grams of concentrated cannabis January 1, 2018
Colorado
medical; effective June 1, 2001 none no more than 2 ounces unless a greater amount is deemed medically necessary
recreational; effective December 10, 2012 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce January 1, 2014
Connecticut
medical; effective October 1, 2012 if under age 21, cannot use medical marijuana that must be smoked, inhaled, or vaporized not to exceed an amount reasonably necessary to ensure the uninterrupted availability for 1 month
Delaware
medical; effective July 1, 2011 if under age 18, may use only medical marijuana oil and only for certain debilitating conditions and symptoms no more than 6 ounces
District of Columbia
medical; effective in 2009 none no more than 4 ounces
recreational; effective February 26, 2015 if under age 21, not allowed to purchase, possess, or use no more than 2 ounces not allowed
Florida
medical; effective March 25, 2016 none 70-day supply unless a greater amount is deemed medically necessary
Georgia
limited medical; April 16, 2015 none no more than 20 fluid ounces of specified cannabis product that contains no more than 5 percent THC
Hawaii
medical; effective June 14, 2000 none no more than 4 ounces
Idaho
no law n/a n/a
Illinois
medical; effective January 1, 2014 if under age 18, may use only medical cannabis infused products and only for seizures or as provided by administrative rule no more than 2.5 ounces unless a greater amount is deemed medically necessary
Indiana
limited medical; April 26, 2017 none specified cannabis product that contains no more than 0.3 percent THC
Iowa
limited medical; effective July 1, 2014 none specified cannabis product that contains no more than 3 percent THC
Kansas
no law n/a n/a
Kentucky
limited medical; effective April 10, 2014 none specified cannabis product obtained pursuant to a written order of a physician practicing at a hospital or associated clinic affiliated with a Kentucky public university having a college or school of medicine, derived from industrial hemp, or approved as a prescription by the FDA
Louisiana
limited medical none 1 month supply; one dose shall contain no more than 10 milligrams of THC
Maine
medical; December 22, 1999 none up to 2.5 ounces
recreational; effective January 30, 2017 if under age 21, not allowed to purchase, possess, or use up to 2.5 ounces delayed until at least 2019
Maryland
medical; effective June 1, 2011 none no more than 30-day supply unless a greater amount is deemed medically necessary
Massachusetts
medical; effective January 1, 2013 none no more than 60-day supply, up to 10 ounces or as determined by the cannabis control commission
recreational; effective December 15, 2016 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce; within the person's primary residence, no more than 10 ounces pending certification of independent testing laboratory
Michigan
medical; effective December 4, 2008 none no more than 2.5 ounces
recreational1 if under age 21, not allowed to purchase, possess, or use no more than 2.5 ounces; within the person's residence, no more than 10 ounces pending regulations, which are due within 12 months of the effective date
Minnesota
medical; effective May 30, 2014 none no more than 30-day supply
Mississippi
limited medical; effective April 17, 2014 none specified cannabis product provided by the National Center for Natural Products Research at the University of Mississippi and dispensed by the Department of Pharmacy Services at the University of Mississippi Medical Center
Missouri
medical; effective December 6, 2018 if under age 18 and not emancipated, may not have an ID card, purchase, or possess marijuana purchase no more than 30-day supply, which can be limited to no less than 4 ounces of dried, unprocessed marijuana, unless a greater amount is deemed medically necessary; possess at least 60-day supply, although qualifying patients who can cultivate marijuana may possess up to 90-day supply
Montana
medical; effective November 2, 2004 if under age 18, cannot smoke medical marijuana and can only used marijuana-infused products up to 1 ounce
Nebraska
no law n/a n/a
Nevada
medical; effective by late November or early December 2000 none no more than 2.5 ounces in any 14-day period unless a greater amount is deemed medically necessary
recreational; effective January 1, 2017 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce July 1, 2017
New Hampshire
medical; effective July 23, 2013 none no more than 2 ounces
New Jersey
medical; effective October 1, 2010 none no more than 2 ounces in a 30-day period
New Mexico
medical; effective July 1, 2007 none 3-month supply
New York
medical; effective July 5, 2014 none no more than 30-day supply; individual dose may not contain more than 10 milligrams of THC
North Carolina
limited medical; effective July 13, 2014 none specified cannabis product that contains no more than 0.9 percent THC
North Dakota
medical; effective April 17, 2017 if under age 19, may use only "pediatric medical marijuana", which contains no more than 6 percent THC possess no more than 3 ounces; purchase no more than 2.5 ounces in a 30-day period
Ohio
medical; effective September 8, 2016 none no more than 90-day supply
Oklahoma
medical; effective July 26, 2018 none no more than 72 ounces of edible marijuana, 3 ounces of marijuana on the person, 8 ounces of marijuana in the residence, and 1 ounce of concentrated marijuana
Oregon
medical; effective December 3, 1998 if under age 18, cannot produce medical marijuana no more than 24 ounces
recreational; effective December 4, 2014 if under age 21, not allowed to purchase, possess, or use no more than 8 ounces of usable marijuana, 16 ounces of cannabinoid products in solid form, 72 ounces of cannabiniod products in liquid form, and 16 ounces of cannabinoid concentrates October 1, 2015
Pennsylvania
medical; effective May 17, 2016 none no more than 30-day supply
Rhode Island
medical; effective July 1, 2006 none no more than 2.5 ounces and an amount of wet marijuana as determined by the departments of health and business regulation
South Carolina
limited medical; effective June 2, 2014 none specified cannabis product that contains no more than 0.9 percent THC
South Dakota
no law n/a n/a
Tennessee
limited medical; effective May 16, 2014 none specified cannabis product that contains less than 0.9 percent THC
Texas
limited medical; effective June 1, 2015 none specified cannabis product that contains no more than 0.5 percent THC
Utah
medical; effective May 8, 20182 if under age 18, may not have a medical cannabis card possess no more than 4 ounces of unprocessed cannabis outside the residence; purchase no more than 2 ounces of unprocessed cannabis in any 14-day period
Vermont
medical; effective July 1, 2004 none no more than 2 ounces
recreational; effective July 1, 2018 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce not allowed
Virginia
limited medical; effective February 26, 2015 none specified cannabis product that contains no more than 5 percent THC
Washington
medical; December 3, 1998 if under age 18, may not grow marijuana plants or purchase marijuana-infused products, useable marijuana, or marijuana concentrates from a marijuana retailer no more than 48 ounces of marijuana-infused product in solid form, 3 ounces of useable marijuana, 216 ounces of marijuana-infused product in liquid form, or 21 grams of marijuana concentrates unless a greater amount is deemed medically necessary,
recreational; effective December 6, 2012 if under age 21, not allowed to purchase, possess, or use no more than 1 ounce of useable marijuana, 16 ounces of marijuana-infused product in solid form; 72 ounces of marijuana-infused product in liquid form, and 7 grams of marijuana concentrate July 8, 2014
West Virginia
medical; effective July 1, 2019 none no more than 30-day supply
Wisconsin
limited medical; effective April 17, 2014 none specified cannabis product in a form without a psychoactive effect
Wyoming
limited medical; effective July 1, 2015 none specified cannabis product that contains less than 0.3 percent THC

1Effective date is dependent on the official declaration of the November 2018 election results.

2Utah's medical marijuana law was originally limited to terminally ill patients. On November 6, 2018, Utah voters approved a broad medical marijuana law.