According to a report in The Independent of London, state-of-the-art gas chromatography mass spectrometry was used to analyze residue from fragments of clay tobacco pipes more than 400 years old from the playwright’s garden.
The report quotes the results of a South African study.
Shakespeare lived at a time when explorers like Sir Walter Raleigh were bringing back samples of plants from around the world. Tobacco, corn (maize) and potatoes were introduced to Europe around this time, and the paper speculated that cannabis might have been regarded as a type of tobacco. The study noted that several plants were smoked in Europe at this time, all referred to as "tobacco." Researchers also found traces of cocaine from the same period, though not in Shakespeare's garden.
No direct connection between the author and the drug have been discovered, but the newspaper claimed to find quotes from Shakespeare's writings that referred obliquely to pot. "In Sonnet 76, Shakespeare writes about 'invention in a noted weed'. This can be interpreted to mean that Shakespeare was willing to use 'weed' (cannabis as a kind of tobacco) for creative writing ('invention')."
"In the same sonnet it appears that he would prefer not to be associated with 'compounds strange', which can be interpreted, at least potentially, to mean 'strange drugs' (possibly cocaine)."