The use of illegal drugs involves many long- and short-term risks. Do not endanger your health: do not use illegal drugs! If, however, you do in fact use illegal drugs, it is advisable to be supported or accompanied by some other person who can call the emergency services in the case of sudden severe illness.

Testing for the presence of illegal drugs

Various tests can be used to detect the presence of illegal drugs in the blood, urine or saliva.

There are two methods of testing:

  • qualitative (saliva, breath)
  • quantitative (blood, urine).

Sometimes testing for illegal drugs is imposed, the used method of sampling depends on the context:

  • roadside testing 
  • testing in the workplace 
  • doping tests in sport 
  • at a road accident
  • at school. 

The greater the quantity of drug taken and the longer it has been taken, the longer the drug remains detectable . Drugs remain detectable longer in urine than in blood.

Qualitative testing

The methods of qualitative testing use a chemical agent, which causes a colour reaction, to verify the presence or absence of drugs in the sample tested: in the saliva and breath (only for alcohol), a positive result indicates overall recent consumption.

Consumption over the long-term can be traced from hair (method used only for testing in specific situations such as antidoping in sport, or in forensic medicine). 

The qualitative tests of saliva only screen for the main drugs and families of drugs: 

Other drugs, such as GHB and hallucinogenic drugs (LSD, hallucinogenic mushrooms), are not easily detected by qualitative saliva tests. 

Most NPSs are only detected by specific laboratory analyses.

Quantitative testing

Quantitative testing methods measure the exact quantity of drugs in the blood, urine or hair samples analysed. A positive result can indicate both recent and previous consumption.

These are very precise methods which can only be implemented in laboratories specialised in analyses. Quantitative methods can detect all known drugs.

Discovery periods

There are discovery periods based on the approximate duration of detectability of a drug according to the type of sample tested (blood, urine, saliva, etc.). 

The table below demonstrates the detection times of the most common drugs.

Substance Urine Blood Saliva Hair


Ecstasy (MDMA)



1 to 3 days

12 hours 



24 hours

10 min up to 3 days Up to 90 days

1 to 5 days (occasional use: 4x/week)

30 to 70 days (1x/day)
72 hours 14 hours Up to 90 days
Cocaïne, crack (Benzoylecgonine)

1 to 4 days (single consumption: 4x/week)

Up to 8 days  (1x/day)
5 hours 24-72 hours Up to 90 days
Heroin 1 to 5 days 6 hours 1 hour up to 3 days Up to 90 days
GHB 12 hours 5 hours / Up to 90 days
LSD 2 to 24 hours 3 hours / Up to 90 days

Testing limitations

Screening tests for drugs can produce results which are contrary to the reality:

  • false negatives: the result of the test is negative when the person in fact has taken drugs
  • false positives: the result of the test is positive when the person in fact has not taken drugs.

False positives can be related to the use of certain medicines which appear in the samples as drugs, or to exposure to cannabis smoke, for example. 

Sciensano collects data and analyses the consumption of drugs, the drug market, the consequences for health, requests for treatment and the policies implemented with regard to drugs in Belgium.

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