I stole this answer from an RN on Yahoo. I cannot vouch for its legal correctness, but as practical advice it makes sense:
"Generally, no. Sometimes, yes.
They CAN drug test you if you are unconscious, and drug testing (tox screen) is necessary to make sure whatever they do doesn't interfere with current drugs or medications in your system; or is necessary to make a diagnosis, rule out drug abuse, etc.
They CAN drug test you if you've signed yourself into psychiatric care; you are generally then required to submit to any required tests/procedures.
They CAN drug test you if there's a suspected drug overdose; without a tox screen they can't treat the overdose.
They TYPICALLY cannot drug test you without a signed consent form from you for any other reason, but you may lose certain privileges if you refuse the drug test.
For example, if the hospital or outside physician is prescribing any type of controlled substances or narcotic meds (for depression, anxiety, pain, or other problems) they are required by law to drug test you to make sure you don't have too much or too little of the narcotic in your system, to prove that you are taking it exactly as prescribed, and are not selling or abusing the meds. Also to prove that you are not dangerously combining them with other street drugs. This is standard, as right now narcotic meds are given out on a "guilty until proven innocent" basis when it comes to abuse, selling, and drug combining. If you refuse a drug test that is ordered for this reason, they can take away any prescriptions they've given you, and/or refuse to prescribe more.
If you have a history of drug abuse, they can request you submit to a drug test before they give you any new medications, to make sure what you're already taking won't interfere with current treatment. However, if you refuse the test, they may refuse to treat you.
HOWEVER; some hospitals have a "blanket" consent form when you first are admitted that you sign, and it states (in the fine print) that you already consent to any testing the doctors deem necessary to diagnose or treat your condition; so it's always helpful to obtain copies of consent forms you signed upon check-in at the hospital. If you did sign one of those, they have the right to test for anything they want. But HIPAA will still cover you as far as privacy - they can't release your information or results to anyone but you without your consent.
If a hospital runs a test without your knowledge, obtain copies of all consent forms you signed and inquire as to the reason they felt it necessary to perform the test. If they did it as a "routine" procedure on a fairly healthy patient (i.e., you weren't in a state of medical emergency) and you did not sign a consent form allowing the test, I would pursue this matter a bit further."
In your situation, it appears like there are a number of issues like drug interaction that would make it a good idea to give consent.
This is not so much a legal issue as a medical issue. In order to provide the proper treatment to you when you go to a hospital, the hospital must know what is in your system and how much there is. There is no way to know this without testing.
The information provided is for general informational purposes only and is not intended to be legal advice. I am only licensed in the Commonwealth of Pennsylvania and I am not providing you with specific legal advice. The law changes frequently and varies from jurisdiction to jurisdiction. Being general in nature, the information provided may not apply to any specific factual and/or legal set of circumstances and/or the jurisdiction where you reside. No attorney-client relationship is formed nor should any such relationship be implied. The information provided is of a general nature is not intended to substitute for the advice of an attorney, especially an attorney licensed in your jurisdiction. Your question, although you may believe is simple, it is not simple. You require legal advice, please consult with a competent attorney licensed to practice in your jurisdiction.
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