Carl Sagan was famous for intoning about “billions and billions” when describing how many stars, galaxies and planets existed in the cosmos. That mantra should be chanted by anyone trying to understand marijuana use and policy in the United States. In the aggregate, Americans report smoking pot about 4 billion days a year (totaling up all the days each individual reported pot use for that year). But under the assumptions that some people understate their marijuana use on government surveys and that some users smoke more than once a day, the true annual total of use episodes could easily exceed 10 billion. These almost inconceivably large numbers lead us to three important realities about pot use.

Reality No. 1: The risks of being arrested for smoking pot are extremely low.

Police made about 620,000 arrests for marijuana possession 2014, which is equal to more than one arrest per minute. This sounds on its face like remarkably aggressive enforcement. But while 620,000 is a large number in absolute terms, relative to even the most conservative estimate of how often Americans use marijuana (4 billion days a year), it’s small. For every marijuana possession arrest in 2014, Americans reported using pot almost 6,500 days. To put it another way, a person who smokes pot once a week faces an average risk of one arrest every 125 years. As the chart shows, that’s a significant decline from the rate of enforcement before the recent wave of decriminalization and legalization policies. But even at the historical height of enforcement Americans did not face even a 1 in 1,000 chance of being arrested when they used marijuana.

Reality No. 2: Marijuana use almost never contributes to suicide or homicide.

Now and then there is a news report on a tragedy in which someone who used marijuana took their own life or that of another person. These sad events typically trigger debates about whether marijuana use had a causal role or whether the death was due to something else, for example other drugs the person was taking or psychiatric problems they might have had. The “billions and billions” rule suggests that such arguments miss a larger point. For example, even a 1 in 250,000 risk of marijuana use causing someone to commit homicide would translate into more marijuana-caused homicides than there are homicides in the United States. This implies that even if marijuana use can truly cause suicidal or homicidal behavior under certain circumstances, the needed combination of factors is as rare as hen’s teeth. Otherwise multiple marijuana-linked homicides and suicides would be reported on newspaper front pages every day of the year.

Reality No. 3: A key impact of pot legalization will be an increase in how much time Americans spend stoned.

As laws relax, prices plummet and a legal industry emerges to promote its product, marijuana use is rising by about 10 percent a year. Taking the billions and billions analogy into account, this is astonishing, because sustaining such a growth rate requires the American population to increase its marijuana consumption by hundreds of millions of use episodes a year. If the typical use episode results in two to three hours of intoxication, that would translate into an annual increase of a billion hours a year or more in how much time Americans spend stoned.

Is adding a billion hours of pot intoxication every year good or bad for the country? Over billions and billions of use episodes, clearly there will be a wide range of outcomes for users, including harmless pleasure. However, drug policy analyst Jonathan Caulkins of Carnegie Mellon University points out that much of U.S. marijuana use isn’t accounted for by healthy, moderate users: “Almost half of marijuana use is by people who meet the medical definition of having a substance use disorder involving marijuana or other drugs.”

The cost of increased enjoyment for some pot users might turn out to be an increase in problems in the lives of people who have preexisting substance use disorders.

Keith Humphreys is a professor of psychiatry at Stanford University.