The Army is dismissing the equivalent of almost a battalion of soldiers every year for taking drugs, a report says.
The Ministry of Defence has a "zero tolerance" towards drugs
The Royal United Services Institute said the number of positive tests for illegal drugs, like ecstasy and heroin, rose from 517 in 2003 to 769 last year.
Positive tests for cocaine use rose four-fold during the same period. A dishonourable discharge is likely after a positive test for illegal drug use.
The MoD said drug abuse was less common among forces personnel than civilians.
Unannounced compulsory drug testing (CDT) is carried out across the Royal Navy, Army and RAF.
In the Army, which tests 85% of its personnel yearly, positive tests rose from 1.4 per 1,000 in 2003 to 4.0 in the first half of 2006 and 5.7 per 1,000 from January to June 2007.
Professor Sheila Bird, a senior scientist with the Medical Research Council writing for the RUSI Journal, said the government had refused to say whether it had changed testing practices since 2003 "on the grounds of cost".
More sensitive tests and more testing after weekends and home leave would "go a long way" to accounting for the rise in positive tests for cocaine, she said.
However, if there had been no changes, she said the cocaine results could signal "a genuine change in soldiers' drug use during a period coincident with major operations".
It could also indicate that cocaine use was actually two to three times higher because there was a high chance infrequent use was going undetected, Professor Bird said.
This was because infrequent use could occur on weekends - but testing may not specifically be carried out on Mondays.
Professor Bird analysed answers to Parliamentary questions to find out about army drug testing.
She said that in 2003 cannabis accounted for 50% of all CDT positive tests and cocaine 22%, but by 2006 the figures were 30% for cannabis and 50% for cocaine.
The switch could be the result of soldiers deliberately moving away from cannabis to "minimise their chance of testing positive" - traces of cannabis remain in the urine for two to three weeks, while cocaine remains for two or three days after use.
Major Chris Lincoln Jones, a former soldier, told BBC Five Live that he was aware that some of his former colleagues had taken drugs.
"A little bit of experimentation goes on, I think, and people fall foul of that."
Major Justin Featherstone, also a former soldier, said the figures did not surprise him because young people often came from a culture where drug use was common and they faced huge stress with tours of duty coming around every 18 months.
But he said the Army's "zero tolerance" approach was not straightforward and that some soldiers shown to be suffering from combat stress and who used cocaine had been allowed to stay in the service.
"Individuals are looked at case by case. It's not some draconian system," he told BBC One's Breakfast.
However, a former chief of staff, Chris Parker, told BBC Radio Oxford that some soldiers who were "not stupid" took drugs to cut short their contract with the Armed Forces.
"Young soldiers if they want to leave the Army have to give a year's notice, and if you take drugs, and you are basically found out by the Army's drug testing programme - which is a regular and random programme that's run - you could be discharged almost immediately," he said.
Drugs 'not tolerated'
But Colonel John Donnelly, head of drug policy for the Army, said it was important to "put these figures in context".
"It's 0.7% of our strength, which compares with over 7% in the civilian work place," he told Breakfast.
"Or put that another way, over 99% of our soldiers get the message and are free from drugs. And that's a very positive message to put out for parents and gatekeepers when we are trying to encourage recruits."
A Ministry of Defence spokesman said drug misuse was "incompatible with service life" and was "not tolerated".
"Positive rates in the Army over the past four years average around 0.77%, compared with over 7% in civilian workplace drug testing programmes in the UK.
"These statistics demonstrate that drug misuse is significantly less prevalent among service personnel than in corresponding civilian demographic groups."
The RUSI report comes a month after 17 soldiers from the Argyll and Sutherland Highlanders were discharged after testing positive for drugs.