Cannabis has been around and consumed since at least the third century BCE, possibly even before this era. The earliest restrictions placed on cannabis was in the Islamic world during the 14th century. In the 19th century, it began to be restricted in colonial countries, followed by restrictions placed worldwide by the 20th century.
Now enter into the 21st century, countries began to re-look at the legalization of cannabis since people on Chemotherapy and those with Aids started reporting reductions in pain from their treatments.
The first to semi-legalize were the Dutch, they were also the first to categorize drugs from the most dangerous to the least. As you can imagine, the least being Cannabis. The rest of the world adopted this categorization and is still used today to determine sentencing guidelines and criminal offenses when it came to possession and use of different drugs.
In 1976 the Dutch allowed recreational use of Cannabis, providing it was used only in coffee shops, possession of up to 5 Grams is no longer a criminal offense and has been completely de-criminalized, however, the local police still have powers to confiscate if deemed necessary, such as at the borders.
In 1996 California followed suit and legalized the use of Cannabis for medical use. Canada followed in 2001 with many medical cannabis prescriptions available and many more dispensaries opening every year. Canada was the first country to adopt a system regulating the use of medical Cannabis.
Countries that also followed Canada include; Portugal (2001), Belgium (2003), Chile (2005), Brazil (2006), Czech Republic (2010).
In the United Kingdom, it is illegal to possess and consume Cannabis, the government moved Cannabis from a class B drug to a class C (which is less severe) in 2004. However, this was moved back to a B in 2009. Medical use was finally passed and legalized in November 2018. Despite the drug being illegal in the UK, they are one of the largest exporters of cannabis in the world. The United Kingdom is home to GW Pharmaceuticals, one of the largest producers of medical cannabis.
We can see from the history that cannabis was used very widely for a wide range of health properties. Towards the 20th century, countries started to illegalize it in an attempt to control and classify public usage. Using modern technology and sciences, we have more information available to us than ever. There has been plenty of rigorous scientific studies that prove medical cannabis can be used to manage a variety of health issues.