Answer: I would imagine that an example of over the counter drugs are. Medicines that are attainable without a doctors prescription. to literally mean that these medicines are available for purchase right off of the store shelves.
Answer: Depends on the kind of drug. It will lessen the effectiveness of many drugs by causing them to move through the body more rapidly. If you mean drugs of abuse, the answer is an emphatic "No!" If anything, it will make the effects worse.
Answer: Im sure there are several. In all my collecting of partially useless information I have found that dubya dubya dubya dot EROWID dot org is an amazing drug resource that spans pharmaseuticals to all naturally occurring intoxicants. Specifically I would think any amphetamine, of which there are many, would show up. Dexedrine is the first to come to mind, or adderall....There are many drugs prescribed for ADD, ADHD, AADD, and other impulse controll behaviors and possibly for things like epilepsy. Its not concrete, but I hope it helped.
Answer: Yes, the medication that you can buy over the counter are in smaller doses. This is to prevent people from overdosing and using strong painkillers when they may not need them. If stronger medications are required they should be prescribed by your GP.
Answer: I believe 50/50. Some OTC drugs work better than Rx ones and vice versa. For high cholesterol I was taking a statin medication - after contracting drug reactions from statins, I switched (with doctors approval) to Niacin which is an OTC item. No ill effects anymore and my cholesterol readings are improving.