Marijuana was criminalized across most of the world in the early parts of the 20th century. There is some confusion as to the reasons as there seems to be different driving forces on either side of the Atlantic.
In the UK, cannabis was outlawed in 1928 after Britain became a signatory to the “International Opium Convention” which was held in Geneva in 1925.
In the US the key law seems to be the 1937 Marijuana Tax Act which was the federal culmination of many separate state laws that had been enacted in the previous years. This may have been in response to lobbying by makers of synthetic fibers that competed with hemp.
Laws usually govern distribution, cultivation, and possession for personal use. Enforcement of the law varies from country to country. Some notable examples include the Netherlands, where cannabis is effectively decriminalized and can be purchased in licensed “coffee shops”.
In many countries, police exercise their discretionary powers to caution users or confiscate cannabis for possession in small quantities that could be deemed for personal use, especially for medical reasons.
A recent example was the declaration by police in Brixton, England, that they would not arrest people for possession of cannabis and instead only issue on-the-spot warnings and confiscate the cannabis. Following this trial, the reclassification of cannabis from Class B to Class C was recommended by the Home Secretary in October 2001. This was then reversed by David Cameron (despite having admitted that he had used it as well as being busted whilst at Eton).
The state of South Australia and the Australian Capital Territory have decriminalized possession of small quantities of cannabis and growing limited numbers of plants for personal use; the government merely charges an on-the-spot fine of A$50. Police interest in personal usage and non-commercial growers in the rest of Australia appears to be limited.
As of early 2000s, Canada and some other countries have started to recognize medicinal use of cannabis separately from “normal” possession. Canada, even recently, decriminalized all Cannabis use, including recreational.