Mood drug

Mood drugs were chemical-releasing cellophane patches used by the people of New Earth from some time before 5,000,000,023 until 5,000,000,053, typically in the New New York Undercity, including Pharmacytown. They generally produced emotions, such as Happy, Anger, and Mellow, although others affected the mind in other ways, such as Sleep, Honesty, Read and Forget. In the Undercity, they were mainly sold by Pharmacist 1, 2, and 3. They were usually harmless.

One mood drug, however, Bliss, was only found in the upper levels of New New York. It was extremely addictive. An airborne virus mutated in it. This spread and killed the entire population of the upper levels within seven minutes, except the Face of Boe in his life-support tank and Novice Hame, whom Boe surrounded with his smoke. The inhabitants of the Undercity and those in the Motorway survived. With no one to feed on, the virus eventually starved to death and became extinct.

A conversation between a Pharmacist and a pale woman suggests some if not all of the mood patches came in different degrees of strengths or duration up to at least forty-three. (TV: Gridlock) Mood patches did not work on the Twelfth Doctor. (TV: Dark Water)

The Doctor's supplies
The Twelfth Doctor kept mood patches aboard the TARDIS. Clara stole what she thought was a Sleep patch from his supply and attempted to incapacitate the Doctor and blackmail him into saving her boyfriend, Danny Pink. The Doctor revealed that the mood patches did not work on him and that he had instead used the patch to put Clara in a dream-state. (TV: Dark Water)

Behind the scenes
In the real world, transdermal patches have been used for the administration of medicine since the 1970s, first being used for motion sickness, before expanding to birth control, nicotine patches, and even caffeine patches, all of which act on mood, just like the mood patches in TV: Gridlock.