Science: Eating a bagel can be bad for your criminal record: Patrick Matthews discovered how poppy seeds confuse drug-testing equipment

This is not quite Confessions of an English Opium Eater, but it is the story of how I became a suspected drug-abuser, and thus potentially unemployable, after I went to my local bakery in north London and bought two poppy-seeded bagels for lunch.

Next day I held in my hand a strip of paper resembling a printout from a supermarket till. It said 'OPIA + POS' and recorded the level of opiates detected in my urine. 'That'll be morphine: more than one milligram per litre of urine,' said Graham Ball, a clinical pathologist at the Chelsea and Westminster Hospital. 'On the way we screen drug abusers I'd have no hesitation in saying that's very clearly positive.'

The poppy seeds used by bakers carry traces from the opium poppy's seed capsule. These contain morphine, though not so that you would notice. The trace is enough, however, to give rise to a damning entry in the personnel file.

For the past two months the Offshore Industry Liaison Committee (OILC), which represents workers in the North Sea oil and gas industries, has been dealing with just such a case involving one of its members.

Kevin Smith (not his real name) had been offered work by Amec Offshore Developments on the Tiffany oil production platform. His offer was conditional on passing an overdue annual offshore medical check-up. Opiates were detected first by Aberdeen Industrial Doctors, a practice that specialises in medical examinations for the offshore industry, and confirmed by Aberdeen Royal Infirmary using high-resolution Gas Chromatography/Mass Spectrometry equipment.

Aberdeen Industrial Doctors asked Kevin whether he had taken any codeine- or morphine-based medicines which might give rise to a misleading result. They did not mention poppy seeds. A spokesman for the practice pointed out that to ask might provide a ready-made excuse for drug abusers.

Kevin was shocked and angry: 'When Amec told me there was morphine in my sample I just couldn't believe it. I spent three months unemployed. I ended up skint.'

He consulted the OILC and was referred to the union's solicitors, who did not consider he had any legal comeback. Some weeks later, he says, he stumbled on a magazine article about a farm which grew poppies for both the pharmaceutical industry and for bakeries, and returned to the OILC offices.

'The lad came back with this theory about poppy-seed bread,' said Ronnie McDonald, the OILC general secretary. 'He's engaged to a girl in Banff up the coast, and he'd eaten this poppy-seed bread from a wee corner shop.'

Mr McDonald submitted samples to Aberdeen Industrial Doctors taken before and after eating poppy-seed bread from the same shop. Sure enough, the second sample was positive. He said: 'It all sounds a bit prankish, but there's a very serious issue involved - particularly as random testing is becoming such a feature of our industry.'

Drug testing is now commonplace, and not only in the oil industry. Some find the trend disturbing. 'We are unhappy at the lack of legal safeguards for those at risk of losing a job because of these tests,' said Connor Foley, of Liberty, the former National Council for Civil Liberties. 'These programmes are operating without any proper legislative framework.'

Test samples are first screened on the relatively inexpensive Emit system. This is loaded with chemical reagents which detect most drugs, and gives a near instant result. A positive result goes to a hospital lab for more detailed analysis.

In the case of a suspected heroin user, Emit will simply register opiates, whereas further analysis can give a full list, including morphine, codeine and a substance only found after heroin use called 6-monoacetyl morphine (6-MAM).

In the United States, but not in Britain, it is standard practice to test specifically for 6-MAM, regarded as conclusive evidence of heroin use. The argument against the test is that 6-MAM disappears as soon as six hours after taking heroin, breaking down into compounds also found in the urine of the eater of poppy-seed bread.

So what can the analysts do when they find opiates but no 'smoking gun' evidence of heroin? Brian Widdop, laboratory director at the National Poisons Unit, carried out large amounts of contract work for Medscreen, one of the largest drug testing companies. 'A company must refer the case to a medical review officer,' he said. 'Does the individual have a history of heroin use? The doctor should discuss it with that individual. There are all sorts of visible signs with drug abuse, and doctors have to use medical know-how and nous.'

Lindsay Hadfield, a Medscreen spokesman, said that a properly conducted drug screening programme must place as much emphasis on reviewing results as on analysis. Medscreen offers courses to medical review officers, but companies are not obliged to sign up for them.

British Rail, a client of Medscreen, recently detected opiates in the urine of one of its staff. He claimed to have eaten poppy seeds. A full medical review followed and the defence was accepted - not surprisingly, as the man was in his fifties and entirely respectable.

Doubtless a young man of counter-cultural appearance would also have received justice. But it is difficult not to feel uneasy about the power put in the hands of doctors - and the authority conferred on machines that can't tell smack from seeded rolls.