We know that marijuana has been legalized in Canada for a few years.

If you're referring to the specific government act, I'd recommend using "it was legalized." Since you are describing an ongoing state, I suggest "it has been legal."

Many local Canadian residents think sugar and fat are more harmful than marijuana, but I personally don't vocal this sentimentdisagree.

"I disagree" is a simpler way to say the same thing.

It cwould be challenging fto support me to testify ity opinion with scientific evidence, because eventhere is no consensus, even among professional doctors are debating it.

I suggest using "would" instead of "could", since "would" is stronger: supporting your opinion will definitely be challenging, according to the second clause of your sentence.

The second and third usages of "it" are vague, so I'd recommend elaborating.

It is true thatWhile marijuana can, to somewhat extent, bring people joy and cure or ease pain, but it can also drive some people into addiction.

On the street, we can see some wobbly homeless guys, many of them; their issues are a result ofrom this medicine, not from sugar.

Many local Canadian residents think sugar and fat are more harmful than marijuana, but I personally don't vocalagree with this sentiment.

You could also use "disagree"

It could beis challenging for me to testify itsupport this with scientific evidence, because even professional doctors are still debating it.

It is true thatWhile marijuana can somewhat bring people joy and cure or ease pain, but it can also drive some people into addiction.

Your original sentence is mostly grammatically correct. These corrections are mostly about flow and making it sound more natural.