There are two major categories for concern over the effects of THC. The first is cognitive and the second is physiological. [Without our brains functioning properly, we can do nothing.] Some effects for concern on the cognitive level are misperceptions and anxiety. Some effects for concern on the physical level are lung damage and decrease in heart rate over time.
Using marijuana also causes other form of physical damage like an increase in heart rate by 20-100 percent. Marijuana users have been estimated to have a 5 times higher chances of dying due to heart attacks one hour after smoking.
Marijuana is usually smoked and it is smoked very deeply. With 50-70% more carcinogens than cigarette smoke regular use is very damaging to the lungs.
Another cause for concern when using marijuana stems from the lack of monitoring and regulations with this drug. Since it is illegal in most places, the amount of THC in each sample of marijuana is often very different. Alcohol, on the other hand is very regulated. A person knows exactly the specific amount of alcohol in each drink. This allows the individual to regulate their level of impairment. The variety of THC levels in marijuana make it impossible to determine the level of impairment before, during, or even after consumption.
I am not an expert on this, having never taken drugs myself, but there does seem to be an increased amount of evidence nowadays on the long-term impact of sustained marijuana use and how it effects thinking and also, in some cases, the ability to socialise and to become accepted by part of a social group. The increased focus on such long-term effects should make us think very carefully about degrading or re-classifying this drug.
From what I have seen and read of marijuana use, it tends to kill motivation. There are probably good reasons that the operative verb is "stoned": we don't expect much of anything from a stone. It just sits there. Maybe some people can use marijuana and still be highly productive and highly focused. The persons I have known who have used it would, in general, not fall into either of those two categories.
THC (Tetrahydrocannabinol) is the oil base of many psychoactive drugs obtained from the Cannabis (hemp plant). Psychoactive simply means that it alters normal brain function and activity. As such, it can cause hallucinations, euphoria, "out-of-body" experiences, dizziness, altered appetite, irrationality, memory loss, depression, suicide, and loss of motivation. Because its toxicity is so low, it has never been known to cause a fatal overdose. It is addictive, though, and long-term use of it is detrimental in the long run.
Repeated use over a period of time leads to THC buildup in a person's fat cells, meaning that the lack of motivation, impaired cognitive ability and judgement, and lack of focus are common for weeks and even months after the person stops. There is some debate about whether or not THC is an addictive substance, or whether or not marijuana use is simply habitual.
Basically, the problem is that THC affects people's ability to do things like concentrating or thinking. It makes it harder for people to learn things and to remember what they have learned. The effects can last for days after the person is no longer "high," which means that a person who smokes often might always be impaired to some extent.
The major cause concern is a criminal conviction. From disqualification for federal monies to subisidize one's higher education to reduced job respects; a compounding effect of moving up in the productive tiers of society due to a criminal conviction is the major cause for concern surrounding illegal use of marijuana/THC.