No, an employer may accept a negative/dilute test result as a negative test. If the employer wants a retest it may do so, however, it must be stated in the employer's policy and the employer must require a retest of all individuals that have a negative/dilute result in the same testing category.
Alcohol affects activity of such neurotransmitters as glutamate, GABA, and dopamine; caffeine blocks the action of the neurotransmitter adenosine. Such changes are studied in the field of psychopharamacology, which concerns the effects of drugs on the mind and brain. This field investigates both recreational drugs such as alcohol, caffeine, marijuana, cocaine, and LSD, and therapeutic drugs such as antidepressants and antipsychotics.
In terms of effects on the body, intoxication with a drug can cause physical effects that range from marked sleepiness and slowed breathing as with intoxication with heroin or sedative hypnotic drugs, to the rapid heart rate of cocaine intoxication, or the tremors to seizures of alcohol withdrawal.