Addiction

Addiction was a dependency on a particular stimulus. According to the Twelfth Doctor, you couldn't tell if something was an addiction until you tried to give it up. (TV: Mummy on the Orient Express)

One could be addicted to a substance, due to excessive use, (PROSE: Unnatural History, Wonderland) such as Vraxoin (TV: Nightmare of Eden) or opium, (TV: The Talons of Weng-Chiang) or potentially to an experience, such as theft, (TV: Fragments) sex or murder. (TV: Kiss Kiss, Bang Bang) Lucy Sobel's boyfriend Ricky was addicted to heroin. (PROSE: Slow Decay)

It was possible to be addicted to time travel. Following her "last hurrah", Clara Oswald was unable to give up travelling in the TARDIS and so chose to lie to the Doctor in order to stay. (TV: Mummy on the Orient Express) Other companions have expressed similar signs of addiction, with both Rose Tyler and Donna Noble stating that - had unforeseen circumstances not broken them apart - they would have travelled with the Doctor "forever". (TV: Army of Ghosts, Journey's End)

Addictions could sometimes be fatal. Bubbleshake was a highly addictive substance containing fabi weed, which caused brain disintegration. Bernice Summerfield became addicted to the substance. (PROSE: The Highest Science)

One might suffer from withdrawal when their intake stopped or slowed down. For Vraxoin, a dangerous, addictive drug, withdrawal symptoms included extreme anger, in contrast to the period of apathy while the drug was in effect. (TV: Nightmare of Eden)

Rehabilitation was an option for dealing with addiction. In the 51st century, John Hart went to rehab for drugs, alcohol, sex and murder. (TV: Kiss Kiss, Bang Bang)

Sarah Jane Smith accused a pair of Slitheen-Blathereen of being addicted to Rakweed, to which Leef Apple Glyn Slitheen-Blathereen denied, retorting that they "just lov[ed] it, that's all." (TV: The Gift)