There are 10 million people in prisons worldwide, 30 million on an annual basis. In Europe an estimated 635,000 (2010) people were in prison, among around 30,000 female prisoners.Many of the inmates are drug dependant.
Drug users are over vastly overrepresented in prisons all over the world. Nearly two thirds of all drug injectors in a 12-city study having been in prison and an Australian study mentiones an average of 5 periods of imprisonment.
In all 5 continents the number of prisoners has increased over the last decades. This is clearly shown in the rates of people in prisoned per 100,000 people (incarceration rate) This figure rose from 136 to 146 per 100.00 over the last 15 years. The frequent and repeated imprisonment of drug users (e.g. due to their dependeny, the illegality of drugs and crimes related to obtaining drugs) is one of the key reasons for the large and increasing number of drug users in prisons. 
Many drug users stop using and or injecting while being in prison. For them the period of being in jail might be a period of stabilisation or rehabilitation of (parts of) their lives. Other continue to use substances on incidental or more regular basis.
According to a study in Belgium one thirds began using an additional substance in prison (in most cases heroin). Todts et al 2008
A comprehensive report by the EMCDDA describes the current situation in Europe on drug use in prisons. Although there are big differences between the countries in Europe, one striking element is that among 9 countries the between 2-4 out of 10 drug users report some kind of continuation during the period in prison.
The health needs of prisoners are diverse and complex. Prisoners suffer from high levels of physical and psychiatric disorders, ranging from infectious diseases (HIV/AIDS, hepatitis B and C, tuberculosis) to psychiatric illnesses (like schizoprhrenia and depression) and substance dependency. 
The health situation of inmates is worse than among the general society and prison situation enhances all sorts of risks and harms. Therefor specific attention is required to be given to the phisical and mental health state of people prisons. European and international prison agreements and rules strongly promote the equivalence of quality and access to care between the prison and the community.
As drugs-related issues are such a substantial and complex area, they are recommended to receive addtional focus and attention. An example of a a reports that highlight this situation is commissione by the EC and written by some European experts.
 EMCDDA. Prisons and drugs in Europe. The problem and the response. 2010
 A. Ball et al. Multi-city study on drug injecting and risk of HIV infection. WHO, 1994
 Dolan et al. HIV risk behavior and preventionin prison. Drug and AlcoholReview 1999.
 European Prison Rules
 Stöver, H., Weilandt, C., Zurhold, H., Hartwig, C. and Thane, K. (2008), Final report on prevention, treatment, and harm reduction services in prison, on reintegration services on release from prison and methods to monitor/analyse drug use among prisoners