If it is a hair follicle test them any and every thing in the past 6 months. Urine usually cocaine, marijuana, heroin, or opiates, meth and barbituates.
This can vary from company to company, both for the drugs themselves or the number of drugs tested for. Many companies use DOT guidelines and test for THC, Opiates, Amphetamines, PCP and Cocaine....
If the prescription drugs are in one of the screened classes of drugs (e.g. opiates, benzodiazepines, stimulants, etc.), then yes, pre-employment drug screens will test for these.