Each drug testing method has certain benefits and drawbacks. Here is an overview of today’s methods and when they are appropriate.

Blood
Blood testing tends to be very accurate, but is rarely used because of its invasiveness.

Hair
Advantages
Timing: With hair testing, you can go back from a few days to three months. It therefore tends to reduce the number of drug-using applicants who apply at your company, since they would have to stay clean for ninety days in order to apply.
Invasiveness: Hair testing is less invasive than blood testing and less embarrassing than urine testing. 
Reliability: Hair testing is more effective than urine testing for detecting hard drugs.
Adulteration Potential: There is no opportunity to adulterate a hair sample.
Retesting: If a test result is disputed, it is easy to get another sample to test.
Disadvantages
Timing: Hair testing does not reveal present use, because it takes about a week for drug residue to appear in the hair. In addition it may show a positive on recovering drug addicts, who are covered by the Americans with Disabilities Act.
Reliability: Hair testing may provide false positives for people who were in the same room with someone smoking marijuana, although some hair testing providers claim to be able to identify external contamination. A hair test may also mistake legal prescription drugs for illegal substances. Finally, hair testing is not as effective as urine for testing for marijuana use.

Urine
Advantages
Timing: Urine testing reveals current use.
Reliability: It is an excellent test for marijuana use.
Retesting: Samples can be divided—split sampled—for later testing if an initial test is positive.
Disadvantages
Timing: Urine testing is only useful for forty-eight hours after drug use. After that, different people will test differently. Some will test positive even if they used same amount of same type of drug as subjects who test negative. As such, it could be claimed to be discriminatory.
Also, while urine is a good test for marijuana use, it can take up to six hours for marijuana to show up in urine. So a sample taken immediately after a workplace accident may not show a positive.
Reliability: Urine is a less effective medium than hair when testing for hard drugs.
Adulteration Potential: There is significant opportunity for adulteration of a urine sample. There are scores of Internet sites offering additive products designed to mask drug use. Approximately 4 percent of all urine samples show adulteration. Even drinking large quantities of water can dilute a urine sample.
Retesting: If there is a dispute, you can test the other half of a split sample, but you cannot collect a new sample, test it, and expect the same results.

Oral Fluid (Saliva)
This new technology involves placing a swab in the mouth for about fifteen minutes until it is saturated with saliva.
Advantages
Timing: It takes only minutes for marijuana to show up in saliva samples and only slightly longer for other drugs.
Invasiveness: The procedure is very convenient, because it can be done anywhere. It is also non-invasive and can be done in front of an observer, unlike urine testing.
Adulteration Potential: To date, there are no known adulterants that work on saliva. 
Disadvantages
Timing: There is a shorter detection period. It cannot detect drugs used more than a day and a half before the test. 
Reliability: Saliva testing does not detect low concentrations of drugs smoked or used intranasally, nor can it detect PCP use.
Retesting: It is difficult to preserve a specimen to retest.

The Right Test
With so many advantages and disadvantages of each type of testing, which makes the most sense? It may be wise to use different tests for different circumstances. 

- Use hair testing for pre-employment tests and to evaluate whether current employees have recovered or been rehabilitated, since it can detect usage back as far as 90 days ago.

- Use urine testing for situations that require knowledge of more current usage, such as for random or periodic testing.

- Use saliva testing for situations that require immediate knowledge, such as post-accident and reasonable suspicion.