The number of middle-aged drug users is on the rise, as many who first experimented during the liberal Sixties and Seventies refuse to give up the habit.
According to research by scientists, cannabis use among the over-50s has increased tenfold since the 1990s.
The number of older people who have taken other illegal substances such as cocaine, LSD and amphetamines has also increased.
A team of academics from King’s College in London compared drug use among 50 to 64-year-olds in 1993 with adults of the same age group in 2007.
They asked 4,000 whether they had taken certain illegal drugs at any time during their life, and if so if they had used them in the past 12 months.
The study found the number of middle-aged people who had taken cannabis in the past year rose from 0.2 per cent in 1993 to 2 per cent in 2007.
And the findings, published in the journal Age and Ageing, also suggest attitudes towards drugs are far more liberal among the middle-aged than the older generation.
Some 11.4 per cent of 50 to 64-year-olds said they had taken cannabis at some point in their lives, compared with just 1.7 per cent of those aged over 65.
Until now it has been very difficult to estimate how many older folk regularly take drugs, as figures are not included in the official British Crime Survey. An annual Home Office survey of drug use only includes adults under the age of 59.
Its most recent figures published in July 2011 showed that three million adults – 8.8 per cent – had used drugs in the previous 12 months.
This included 2.2million who had used cannabis, 700,000 who had taken cocaine and 500,000 who admitted to taking ecstasy.
But overall the number of adults using cannabis has fallen in the last decade, particularly among the 16 to 24 age group. Experts believe this is due to a change in attitude among youngsters who are more aware of its health risks.
However, survey author Professor Robert Stewart predicted illicit drug use among pensioners will become increasingly common as adults who regularly took substances in their youth grow older.
He said: ‘The key message of this paper confirms something which has been long-suspected but which has not, to our knowledge, ever been formally investigated in the UK – namely that illicit drug use will become a more common feature in older generations over the next one to two decades.
‘These are generations of people who were exposed to drugs. It is less of a taboo subject – it is more acceptable.’
He said doctors needed to be aware of potential drug use among patients of this age and their possible health risks.
Professor Stewart also said more research was needed to ascertain whether adults in their 50s and 60s were more likely to suffer side-effects from these substances.
‘One particular issue is that we really know very little about the effects of drugs like cannabis in older people but will need to work fast if research is to keep up with its wider use at these ages,’ he added.
Cannabis use can raise the heart rate, increase blood pressure and cause feelings of anxiety and paranoia. Regular use has also been linked to schizophrenia and other mental health problems.
In 2009 cannabis was reclassified from a Class C to a Class B drug, after ministers concluded it posed a risk to long-term health.
The change in status meant anyone caught possessing the drug faced a maximum jail term of five years, rather than two.