From Wikipedia:
McCartney first used drugs in the Beatles’ Hamburg days when they often used Preludin to maintain their energy while performing for long periods. Bob Dylan introduced them to marijuana in a New York hotel room in 1964; McCartney recalls getting “very high” and “giggling uncontrollably”. His use of the drug soon became habitual, and according to Miles, McCartney wrote the lyrics “another kind of mind” in “Got to Get You into My Life” specifically as a reference to cannabis. During the filming of Help!, McCartney occasionally smoked a joint in the car on the way to the studio during filming, and often forgot his lines. Director Richard Lester overheard two physically attractive women trying to persuade McCartney to use heroin, but he refused. Introduced to cocaine by Robert Fraser, McCartney used the drug regularly during the recording of Sgt. Pepper’s Lonely Hearts Club Band, and for about a year total but stopped because of his dislike of the unpleasant melancholy he felt afterwards.
Initially reluctant to try LSD, McCartney eventually did so in late 1966, and took his second “acid trip” in March 1967 with Lennon after a Sgt. Pepper studio session. He later became the first Beatle to discuss the drug publicly, declaring: “It opened my eyes … [and] made me a better, more honest, more tolerant member of society.” He made his attitude about cannabis public in 1967, when he, along with the other Beatles and Epstein, added his name to a July advertisement in The Times, which called for its legalisation, the release of those imprisoned for possession, and research into marijuana’s medical uses.
In 1972, a Swedish court fined McCartney £1,000 for cannabis possession. Soon after, Scottish police found marijuana plants growing on his farm, leading to his 1973 conviction for illegal cultivation and a £100 fine. As a result of his drug convictions, the US government repeatedly denied him a visa until December 1973. Arrested again for marijuana possession in 1975 in Los Angeles, Linda took the blame, and the court soon dismissed the charges. In January 1980, when Wings flew to Tokyo for a tour of Japan, customs officials found approximately 8 ounces (200 g) of cannabis in his luggage. They arrested McCartney and brought him to a local jail while the Japanese government decided what to do. After ten days, they released and deported him without charge. In 1984, while McCartney was on holiday in Barbados, authorities arrested him for possession of marijuana and fined him $200. Upon his return to England, he stated: “cannabis is … less harmful than rum punch, whiskey, nicotine and glue, all of which are perfectly legal … I don’t think … I was doing anyone any harm whatsoever.” In 1997, he spoke out in support of decriminalisation of the drug: “People are smoking pot anyway and to make them criminals is wrong.” He did, however, decide to quit cannabis in 2015, citing a desire to set a good example for his grandchildren.
Paul McCartney: Ten Days in the Life
Feb 20, 1980 • From RollingStone
Jun 19, 1967 • From ITN
Jul 19, 1967 • From Queen Magazine
Beatle Paul: I admit danger of LSD
Jun 19, 1967 • From Daily Mirror
LSD: Mirror doctor raps Beatles Paul
Jun 18, 1967 • From Daily Mirror