WEED
There are a large number of websites available for anyone to research the pros & cons of marijuana use for Medical purposes and Recreational uses.
There is also a start to the research database on this topic (in other words, not many research studies) that look at the use of marijuana in the medical field.
But there really is not a great deal of research looking at the effects of marijuana for recreational purposes. Any claims that marijuana causes no harm or does a lot of harm really needs to be backed up by multiple, well-thought-out, peer-reviewed in scientific journals, research studies (not just one or two... which many of the above mentioned websites use as "evidence" to support their claims).
What we do know:
1) Marijuana can affect short term memory. A student, whose goal is to do well, wouldn't go and drink alcohol before going to class. Students should not also go and do the same with Marijuana (and I have noticed over my years at MU that a few students (very small number) have come to class after taking a hit or more). Marijuana affects short term memory. That is, a person will be less likely to remember (a class lecture) or recall information (such as for a test) if they are under the influence of any level of marijuana. Even a PRO marijuana websites agree on this memory loss http://www.drugpolicy.org/drug-facts/10-facts-about-marijuana (when on the page, click the "Does marijuana use have long term effects on the brain?" link).
2) There is more THC in a joint now compared to just a few years ago. So when older people say that it really didn't affect them when they were younger, they might be right because of the lower THC. Even state-regulated marijuana does not monitor the exact amount of THC. So the effects could be greater with higher levels of THC leading to a possible addiction. https://www.cnn.com/2022/07/25/health/marijuana-potency-addiction-study-wellness/index.html
3) Long term issues have also been a topic for research studies in marijuana use. A study from 2014 demonstrated that long term effects of pot can shrink a part of the brain. http://www.cbsnews.com/news/what-smoking-marijuana-does-to-the-brain/ Researchers are still trying to figure out what that does to brain function. But I don't think intelligent people would think that shrinking the brain would be good in any sense?
4) Weed, when smoked, may increase your chance for cardiovascular diseases. The oral or topical forms of cannabis may reduce some of this potential harm: https://www.cnn.com/2020/08/05/health/weed-marijuana-heart-wellness/index.html
5) Synthetic Marijuana is just bad. Click Here It is illegal (but so is regular marijuana in most states). It is not regulated by anybody... so who knows what chemicals people get in products like "Spice", "K2", "Black Mamba", etc. People who advocate for marijuana legalization hate that this product is called synthetic marijuana because it's effects are nothing like "normal" marijuana. And the effects of the synthetic version are well documented, including many deaths.