The "Gateway Theory" is a theory that the use of cannabis naturally leads to the use of heroin, cocaine, or other "hard drugs". This theory is often used to explain why cannabis use must remain criminal, despite cannabis use itself being less physically dangerous than many legal activities. Although the Gateway Theory has been repeatedly debunked and discredited by reviews funded by the White House, the Parliament of Britain, and the government of Canada, it continues to be used as an argument in both educational materials and political decisions.

Based on the data from the National Survey on Drug Use and Health (NSDUH), formerly named the National Household Survey on Drug Abuse, by SAMSHA, the percentage of those who have ever tried marijuana and then ended up using heroin once per month is around 1 in 1000. To explain this further, since the estimated number of monthly heroin users in the United Status in 2001 is 100,000, that is the maximum number of people who can have gone on to use heroin regularly after trying cannabis. Since there are an estimated 83 million people in the US who have ever tried cannabis, this works out to 0.12% (100,000 / 83,300,000).

The chart below also helps show the correlation between those who ever try a substance and then go on to ever use it at least once a month. Of the psychoactive drugs included in this survey, trying PCP was the least likely to lead to ever using it monthly (0.8%), the next least likely to lead to any monthly use was LSD (1.5%), next were inhalants (2.7%), then heroin (3.2%), then barbiturates (4.1%).

2003 Data

Percent of people who ever used a drug and then kept using the drug at least once a month
(Data from 2003 National Survey on Drug Use and Health, SAMHSA)
drug	used ever (millions)	used 1X/month (millions)	use ranking		ever used and kept using 1X/month	ever used but NOT using 1X/month	single use MOST likely to lead to regular use	Single Use Least Likely to lead to regular use
alcohol	197.5	119.0	1		60.3%	39.7%	1
cigarettes	163.2	60.4	2		37.0%	63.0%	2
marijuana	96.6	14.6	3		15.1%	84.9%	3a
cocaine (all)	34.9	2.3	4		6.6%	93.4%	6
hallucinogens (all)	34.4	1.0	5		2.9%	97.1%	12
pill opioids	31.2	4.7	6		15.1%	84.9%	3b
hallucinogen: LSD	24.4	0.1	7		0.5%	99.5%	15	1
inhalants	23.0	0.6	8		2.6%	97.4%	13	3
stimulants (all)	20.8	1.2	9		5.8%	94.2%	7
benzodiazepines	20.2	1.8	10		8.9%	91.1%	4
stimulant: meth	12.3	0.6	11		4.9%	95.1%	8
hallucinogen: MDMA	10.9	0.5	12		4.3%	95.7%	10
barbiturates	9.5	0.3	13		3.2%	96.8%	11	4
cocaine: crack	7.9	0.6	14		7.6%	92.4%	5
hallucinogen: PCP	7.1	0.1	15		0.8%	99.2%	14	2
heroin	3.7	0.2	16		4.6%	95.4%	9

2002 Data

Percent of people who ever used a drug and then kept using the drug at least once a month
(Data from 2002 National Household Survey on Drug Abuse, SAMHSA)
drug	used ever (millions)	used 1X/month (millions)	use ranking		ever used and kept using 1X/month	ever used but NOT using 1X/month	single use MOST likely to lead to regular use	Single Use Least Likely to lead to regular use
alcohol	195.5	119.8	1		61.3%	38.7%	1
cigarettes	162.6	61.1	2		37.6%	62.4%	2
marijuana	95.0	14.6	3		15.4%	84.6%	3
hallucinogens (all)	34.3	1.2	4		3.5%	96.5%	12
cocaine (all)	33.9	2.0	5		5.9%	94.1%	7
pill opioids	29.6	4.4	6		14.9%	85.1%	4
hallucinogen: LSD	24.5	0.1	7		0.4%	99.6%	15	1
inhalants	22.9	0.6	8		2.6%	97.4%	13	3
stimulants (all)	21.1	1.2	9		5.7%	94.3%	8
benzodiazepines	19.3	1.8	10		9.3%	90.7%	5
stimulant: meth	12.4	0.6	11		4.8%	95.2%	9
barbiturates	10.0	0.4	12		4.0%	96.0%	11	4
cocaine: crack	8.4	0.6	13		7.1%	92.9%	6
hallucinogen: PCP	7.4	0.06	14		0.8%	99.2%	14	2
heroin	3.7	0.17	15		4.6%	95.4%	10	5

2001 Data

Percent of people who ever used a drug and then kept using the drug at least once a month
(Data from 2001 National Household Survey on Drug Abuse, SAMHSA)
drug	used ever (millions)	used 1X/month (millions)	use ranking		ever used and kept using 1X/month	ever used but NOT using 1X/month	single use MOST likely to lead to regular use	Single Use Least Likely to lead to regular use
alcohol	184.4	109	1		59.1%	40.9%	1
cigarettes	151.6	56.2	2		37.1%	62.9%	2
marijuana	83.3	12.1	3		14.5%	85.5%	4
hallucinogens (all)	28.3	1.3	4		4.6%	95.4%	9
cocaine (all)	27.8	1.7	5		6.1%	93.9%	8
pill opioids	22.1	3.5	6		15.8%	84.2%	3
hallucinogen: LSD	20.2	0.3	7		1.5%	98.5%	13	2
inhalants	18.2	0.5	8		2.7%	97.3%	12	3
stimulants (all)	16.0	1	9		6.3%	93.8%	7a
benzodiazepines	13.9	1.4	10		10.1%	89.9%	5
stimulant: meth	9.6	0.6	11		6.3%	93.8%	7b
barbiturates	7.4	0.3	12		4.1%	95.9%	10	5
cocaine: crack	6.2	0.4	13		6.5%	93.5%	6
hallucinogen: PCP	6.0	0.05	14		0.8%	99.2%	14	1
heroin	3.1	0.1	15		3.2%	96.8%	11	4