Drugs can cause addictions. Addictions are diseases by RAW so i would handle drugs like diseases.
I would say so, yes.
I'd say yea. Most drugs are just very specific or very mild poisons that affect the brain in a specific way. And most drugs work exactly the same as ingested poison (save DC, onset period, ability damage, etc).
Following strictly RAW the answer is NO. Drugs are alchemistic substances and not detectable by any core spell.
Possible undesirable effects of drug usage are addictions and addictions are diseases by RAW. You can detect a drug addict with 'Diagnose Disease' and cure him 'Remove Disease'.
On my table i would allow 'detect Poison' for drugs but only for heavy (not legal) drugs. Alcohol or nicotine are drugs too they are legal in 90% of the world. Sugar can be a drug .. everything that provokes an endorphin push is some kind of drug and you can become addicted to that.
Technically, they aren't poisons, though they probably should be. Drug addiction is treated as a disease, though.
Only if detect poison also detects alcohol.
Can you use detect poisons to detect drugs?
I would say yes. But the argument that drugs are just diluted poisons, tehrefore detectable by detect poison, would also mean that most herbal remedies would detect as poison. IRL, most herbs, used to treat illnesses, are used very sparingly. Thinks like monkshood and arsenic (OK, not an herb...) are still used to treat illnesses in many areas of the world. There is a therapeutic threshhold and a toxic threshhold. That, and if you add in the homeopathic side of things, it gets even mroe involved.
But for the simplicity of Pathfinder, I would say yes...that heroin detects as poison.
Only if detect poison also detects alcohol.
Since you can suffer from alcohol poisoning, yes.