Prof Iversen believes cannabis has been "incorrectly" classified
Professor Les Iversen is the new interim chairman of the government's Advisory Council on the Misuse of Drugs. That means he will be holding the ring amid an ongoing row over politics and scientific advice.
The retired Oxford University professor of pharmacology is a specialist in neuropharmacology, the study of how drugs or chemicals affect the brain and nervous system.
He has published more than 350 scientific papers and is a fellow of the Royal Society and a foreign associate member of the US National Academy of Sciences. He is on the board of directors for a company specialising in developing drugs for nervous system disorders.
Prof Iversen has a long-standing advisory relationship with government and parliamentarians and steps up to the post of ACMD chairman after having earlier been the head of the body's technical committee.
In 1998, he acted as the key specialist adviser to a report into cannabis by the House of Lords' respected science and technology committee.
It concluded that the government should allow doctors to prescribe cannabis for medical use - and that the law should not stand in the way of relieving pain, particularly for people with multiple sclerosis.
At the same time, it concluded that the ban on recreational use of cannabis was justifiable because of the evidence of the harm it did to healthy people.
Professor Iversen joined the ACMD itself in 2004 and was also a member of the Royal College of Physicians' inquiry that in 2006 argued that doctors should conduct further research into the medicinal properties of cannabis.
He has also published two books on the science of drugs. In the first,
The Science of Marijuana
, he details the advances made in understanding marijuana and the balance between benefits and risks of using the plant in medicine. He also describes how scientists have found substances similar to cannabis that naturally occur in the brain in specific circumstances.
He warns in the book against politicians using expert inquiries into cannabis to avoid having a proper debate, arguing that scientific evidence into its effects has to be at the heart of public understanding.
"It is not the purpose of this book to persuade the reader to join one side or the other of the cannabis wars but rather to seek some middle ground in this debate that has become so polarised.
"By the end of the 20th Century we have reached an interesting stage in the cannabis debate in the Western world. We must soon decide whether to reintroduce it into our medical cabinets and whether to accept, albeit grudgingly, that the recreation use of cannabis has become part of our culture."
Cannabis has never been legalised for medicinal use in the UK - but Prof Iversen has been at the heart of the debate over its classification.
He was a member of the committee when it recommended downgrading cannabis from class B to class C in 2004. He was still a member when the former Home Secretary Jacqui Smith reversed that decision in 2008.
The new chairman of the ACMD faces a number of immediate challenges. First, the body's work has fallen behind schedule because of the row and subsequent resignations over Professor David Nutt's sacking.
With cannabis now firmly back as a class B drug, the committee is increasingly turning its attention to the rapidly changing marketplace of so-called "legal highs" of manufactured drugs.
Three groups of psychoactive substances were classified at the end of 2009 and the committee is now looking at mephedrone, also known as Meow or Toot.
Although the committee's investigations are continuing, it has already written to the home secretary warning of the possible dangers of the drug and five other similar compounds.
At the same time, Prof Nutt is launching his own rival drugs body, which includes other eminent scientists and will lobby for changes based on its expert opinions.
There are few differences in the scientific opinions held by Prof Nutt and Prof Iversen. But the new chair has already said that it was his duty to provide the best advice possible - but it was the prerogative of ministers to decide whether or not to accept it.