Smith, unlike Anna the sassy barrister in upmarket soap opera This Life, has not been caught. He says that his colleagues are unaware of his drug usage, but that he knows other lawyers who also take stimulants either for pleasure or to help them cope with the stress of the job.
There are no figures available for drug abuse in the workplace, but anecdotal evidence suggests that it is on the increase. Last week the London Evening Standard ran a front-page story headlined "Warning on addict lawyers" which revealed that a help line set up for solicitors showed alarming levels of cocaine and heroin addiction.
But a spokeswoman for the Law Society says: "I think that was a lot of hype to do with the popularity of This Life. We set up a counselling service in May to help and support those solicitors who feel they need to talk to someone about their problems. So far the service has received 41 calls. Many of them are depression related, some are drink related but only two have been drug related - there are 87,000 solicitors in Britain."
The Bar Council says that it will not comment on drug usage among barristers, but a spokeswoman says "they are human beings like the rest of us".
During the Eighties it was the City that gained a reputation for drug- taking and excess, and some of that old-style boozing and coke-snorting is returning, says Andy West, a City trader.
"In the Eighties some City boys certainly boosted the Colombian economy and I did a bit myself. Now it's starting to happen again and I'm seeing the younger ones doing a lot of coke. They have very stressful jobs and drugs give them a buzz and tremendous energy, but part of the problem is that most of them have far too much money and don't know what else to spend it on," he says.
But it's not just the high earners who are taking drugs in the City. Alison Price is a secretary and says "Although I don't take anything at work because it's not worth the risk, there are times when if I've been out clubbing during the week I'll go into work the next morning still a bit out of it. But I don't think that's any worse than my boss coming into work with a raging hangover because he's had a bottle of Claret the night before."
Drug usage at work may be far more prevalent than employers realise, says the Institute of Personnel and Development (IPD). It commissioned a survey of 1,500 personnel professionals who revealed that 15 per cent of their organisations had received reports of employers using illegal drugs in the past year.
"These figures could represent the tip of the iceberg as far as actual levels of drug abuse by workers are concerned," says IPD policy adviser Oonagh Ryden. She believes that the problem often goes unnoticed by management.
"A coke addict is not going to do a line in full view of colleagues, although a heavy drinker may go to the pub with their manager because alcohol is legal and therefore socially acceptable," she says.
The IPD says that employers can't afford to be complacent about substance abuse at work, whether it is alcohol, illegal or prescription drugs. It says that research from the US indicates that employees who use drugs are a third less productive than their colleagues, 3.6 times more likely to injure themselves or somebody else in a workplace accident and 2.5 times more likely to be absent from work for eight days or more.
According to figures published by the American Management Association, 81 per cent of big companies now subject employees to drug testing either before they join the company or on a random basis. In Britain it has usually been confined to industries such as transport or oil, where safety is paramount. But Oonagh Ryden suggests that "testing may also need to be introduced for other jobs where drug abuse may impair performance, such as the City, where the wrong decision from a drugged-up dealer could result in the loss of millions of pounds". There is evidence that a number of City institutions are testing employees for drugs - usually as part of a medical test when they join the company or if there is cause for concern about a particular individual.
One City drugs testing unit says: " We advise City institutions on health issues and five or six of them are doing drug-screening pre-employment."
A spokesman for Chase Manhattan Bank says: "It's not something we want to discuss although new entrants are tested but that is pretty standard."
Another City source says: "It's mainly American companies in the City which do drug-testing because it's pretty routine in the States. But the general feeling is don't test because you could get into a terrible dilemma - what do you do if you find out that someone is using drugs but is a really high performer? It's an issue they would really rather not think about."
Testing employees for drugs is a minefield. Ms Ryden says that unless tests are carried out at frequent intervals after employment, a drug abuser can simply pick up the habit where they left off to avoid pre-employment detection.
But more worrying is the fact that drug tests are not foolproof, as the case of athlete Diane Modahl - who failed a drugs test and was then cleared - illustrates. In the US there are a growing number of cases where employees who have tested positive for opiates are subsequently found to have taken nothing stronger than a poppy seed bagel at lunch time.
Employers organisations say that in any case testing is pointless unless companies have established a clear policy on how to deal with drug usage.
Greater Manchester Police Force, in conjunction with the TUC and the Institute of Directors, among others, is launching a drugs awareness package for employers that contains placebo drugs so that they can recognise illegal substances if they come across them. Chief Inspector David Williams says: "This is based principally on a welfare basis. It's about removing drugs from the workplace, not the individual from the workplace. We are urging employers to get a policy in place that can help individuals with drug problems." Ms Ryden adds that "the emphasis should be on rehabilitation with dismissal as a last resort. It's not in the employer's interest to lose talented people".
Meanwhile Mark Smith says: "My use of recreational drugs is not affecting the quality of my work, but if ever I knew I were to be subjected to a drug test then I would give up and steer clear of the smoked salmon poppy seed bagels, too. I love my job too much to lose it"
Some names have been changed.