There are rumours that the United States federal government may soon lift its ban on cannabis, but that wouldn’t end cannabis prohibitions in America. This incongruity is the result of federalism: the ability of each jurisdiction — the federal government and every state — to maintain its own laws as to which drugs are illegal and which are not.

Completely legalising cannabis in the United States would require the actions of both the federal government and every state government. If the federal government repealed its criminal prohibition of cannabis or rescheduled the drug under federal law, that would not change state laws that forbid its possession or sale. Likewise, state governments can repeal their cannabis laws, in whole or in part, but that does not change federal law.

When Colorado and Washington legalised the possession of less than one ounce of cannabis, questions arose as to how this would interact with federal law. Specifically, the question was whether such state efforts are pre-empted by the federal law, which still prohibits cannabis as a controlled substance like heroin and cocaine.

The answer is clear: states can have whatever laws they want with regard to cannabis or any other drug.

No state is required to have a law prohibiting or regulating cannabis. The Supreme Court has repeatedly held that Congress cannot force states to enact laws; such coercion violates the Tenth Amendment. A state could choose to have no law prohibiting cannabis, or a law prohibiting cannabis with an exception for medical use, or a law allowing possession of small amounts of cannabis, or anything else. In fact, across the US today, this is exactly the situation — many states have very different laws concerning cannabis.

Similarly, if the federal government were to repeal the prohibition of cannabis or reschedule it under the Controlled Substances Act, that would not change state laws. States still could prohibit and punish the sale and possession of cannabis under state criminal statutes. Contrary to what many believe, cannabis laws continue to be enforced by both states and the federal government. According to statistics from the Federal Bureau of Investigation, 693,482 individuals in the US were arrested in 2013 and charged with cannabis violations. Of these, 609,423 — or 88 per cent — were arrested for simple possession. There is an enormous cost in terms of law enforcement resources, the criminal justice system and people’s lives for cannabis to remain illegal.

Even for those arrested and never prosecuted or convicted, arrest records have real harms in terms of the ability to get jobs, loans, housing and benefits.

Like all drug laws, the prohibition against cannabis is much more likely to be enforced against African Americans and Latinos than against whites. According to a 2013 study, whites and blacks use cannabis at roughly the same rates, but blacks are 3.7 times more likely than whites to be arrested for possession of cannabis.

Yet there is little benefit to illegality. The primary argument for keeping cannabis illegal is that it is harmful. But as President Barack Obama observed, pot is no “more dangerous than alcohol”. Many things are harmful — cigarettes, foods high in sugar and salt, and cholesterol — but that does not mean that they should be illegal. In fact, there is a good deal of evidence that cannabis is significantly less harmful than tobacco or alcohol and that it has benefits in treating some medical conditions such as glaucoma and seizure disorders, and alleviating some of the ill effects of chemotherapy. That is why 24 states and Washington DC allow medical use of cannabis. Like the prohibition of alcohol in the 1920s, the prohibition of cannabis has been a failure. The drug is readily available and it is estimated that 30 million Americans used it in the past year. And similar to the prohibition of alcohol, it is a costly failure. In addition to the cost in enforcing the criminal laws, there is the loss of significant revenue that could be gained from taxation and legalisation.

It is a question of when, not whether, cannabis becomes legal in the US. A study by the Pew Research Centre last year found that a majority of Americans now favour legalisation and only 44 per cent believe that it should be illegal. Of those under 35 years old, 68 per cent believe that cannabis should be legal. But there is no doubt that the confusion federalism entails will make legalising cannabis much more difficult.

•Erwin Chemerinsky is dean and distinguished professor of law at the University of California, Irvine School of Law