What is the main cause for drug abuse?

You are going about this backwards The NUMBER ONE reason students - high school or college - have difficulty writing papers is that they try to write without first LEARNING about the topic. You need to conduct preliminary research BEFORE you begin writing.

Find 3-5 academically-acceptable resources that offer information and/or opinions on the subject, and do AT LEAST 3 hours of research, taking notes as you go. Then, try again to define the main idea(s) of your speech

Good luck.

1) Drugs, 2) Abuse, 3) Drug Abuse.

How people live and their decisions.

Childhood trauma can be a reason. 2 brothers I went to school with both became heroin addicts and I found out their mum used to beat them when they were children.

Addiction is like a known trap that people keep willfully walking into.  Take vaping for example, people know that nicotine is addictive, but they willfully get addicted anyway.  Those people must have a mental impairment.

Both the groceries question and this question are fairly easy (at least, to form an opinion on), it's just a question of how much actual evidence you can dig up and phrase in the proper way.

E.g. for the groceries thing: cost goes up since cost of transfer went up? Why did that go up? Due to cost of oil going up? Why did the cost of oil go up? Because X country doesn't want to do business with us or because there was bad PR in the media hence the oil related stocks took a tank, etc.

In regards to drug abuse, three thoughts come to mind, they could all be the same: escapism, pursuit, support. I.e. the first is fairly obvious, to the drug allows one to escape one's current situation.  The second, pursuit, is more carnal. I.e. an obsession over the pleasure derived from the substance. Lastly, support can be viewed as it being a fixed  constant. E.g. during any difficult situation you can always smoke (or drink) and cast aside your worries.Obviously, these are just concepts. You would then have to find evidence that supports these as true reasons people use drugs. This could come from testimonials or from studies done on the effects of certain substances.

If useful, hopefully you cite me in your speech/paper--in vocalized MLA format.

What research have you done? Do you have any ideas of your own?

Well, one big reason is that people start with legitimate medical problems that are corrected, but become addicted to prescription medications during their course of treatment. When they get cut off from the legal drugs, they seek them out illegally.

Is it one of the top three reasons? I have no clue. But I'm a nurse in a hospital and it's extremely common.