Dependency.

'Britain has a new drug problem: two million of us are addicted to tranquillisers, while countless thousands abuse over-the-counter painkillers.
The most common addicts are women aged 18-36 ...
... Although all drugs can cause side effects, two types of legal drug are habit-forming:
· Benzodiazepines (tranquillisers)
· Opiate-based painkillers (such as morphine and codeine)
Both can have serious, even fatal, consequences for users.
Experts say that addiction to prescription drugs is a real problem and one that GPs need to be made more aware of.'

LINK: http://www.gm.tv/lifestyle/health/38271-prescription-addiction.html