Taking legally prescribed drugs does not mean you fail a drug test and it also doesn't mean you get a free pass on them.
There are many drugs listed in Schedules 4 and 8 of the SUSMP which are available for a Medical Practitioner to prescribe that may trigger a positive result on a drug test.
THC - Can be triggered by a legal prescription of cannabis.
MET/AMP - Can be triggered by legally prescribed stimulants/amphetamines such as ADHD medication or relatively high doses of Pseudoephedrine.
OPI - Can be triggered by some lawfully prescribed painkillers.
OXY - Can be triggered by a prescription of Oxycodone
BZO - This is the most common to be tripped and can happen from a person who is taking prescribed sleeping tablets as directed.
These are just a few examples. Many drugs from anti-depressants to non-steroidal can trip a drug test. But this is why only shonky employers or drug testing cowboys will say you have 'failed' a drug test at the field testing stage. Laboratory workers are health/medical employees unlike drug testing officers and they will determine if you are compliant with your medication.
How can I protect myself?
If you are taking any drug, particularly if you suspect it may impact the drug testing results, you should tell your testing officer at the start of the test. If you have the prescription/pack with you then providing it is best, otherwise the drug name/frequency/dosage should be provided.
The reason for this is a laboratory can calculate your dose in oral fluid/urine/blood and compare it with the amount prescribed and see if you are taking your medication in a compliant manner or are taking an excessive dose.
Consequences of exceeding prescribed dose
The consequences of exceeding a prescribed dose is that a regulatory body (i.e. AHPRA), employer or police can view the result as if you took an illegal drug. If the medication is not having a desired effect, this is something that needs to be discussed with a medical practitioner, not dealt with by self prescribing and administering drugs.
Can I still drive with a drug in my system?
In the case of cannabis, sadly in NSW and Queensland - even if the drug is legally prescribed you cannot drive whilst it is in your system. For all other prescribed drugs, provided you are adhering to the conditions of your prescription, you do not feel impaired and no medical practitioner has advised you not to drive whilst taking the medication then you can legally drive whilst prescribed a drug.
The same is generally true for attending work. Provided you do not feel impaired, have not exceeded your prescribed dose and a medical practitioner has not advised you to cease work, then an employer cannot prohibit the usage of a lawfully prescribed drug.
What happens when you test not-negative due to a prescribed drug?
This depends on policy and circumstances. In some cases a company may request that you have the results confirmed by a laboratory, in other cases they may just disregard any not-negative result that is likely caused by a prescribed drug.
If you want to find out more, contact your workplace Safety Officer or your manager.