Many agencies and employers use hair when performing drug screens. Evidence of drugs can stay in hair indefinitely. Stopping the use of a drug for weeks or longer prior to a drug test using hair does not stop the drug from showing up.
Employers have the right to not hire or fire those that test positive for drugs. Many mistakenly believe if you live in a state where medical or recreational marijuana is legal a employer cannot fire you if you test positive for THC. This is not true.