Marijuana is relatively nontoxic. Nobody has ever died of overdosing on marijuana, though it is theoretically possible. Combining marijuana with other drugs can lead to overdose death, and combining marijuana with driving or other activities which require fast reaction time has undoubtedly resulted in trauma related death. Still, the chemicals, including tetrahydrocannabinol, which cause marijuana's high and helpful effects, are mostly not terribly harmful.
Marijuana can be smoked, in which case its effects are noticed quickly and last for 2-3 hours, or taken orally, in which case effects can be delayed for hours and can persist for quite a long time as the drug is more gradually absorbed. Smoking is a particularly good delivery method from the standpoint of a pharmaceutical because of the quick onset which means that a person is more able to accurately judge the appropriate dose, titrating to the desired effect.
Mentally, marijuana can cause anxiety and even paranoia. Usually, though, it is more likely to be sedating than anything else. It can cause euphoria and perceptual distortions. It interferes with formation of memory, which makes it a bad choice for students. It is often good for treating anxiety and sleeplessness, is especially good for nausea and relieves various kinds of pain, including the pain of fibromyalgia (a brain related sensitization to bodily pain with associated symptoms of sleep disorder, irritable bowel, headaches and sometimes confusion) which is difficult to treat with other pain medications. It can significantly reduce the need for opiate pain medications in patients with chronic pain, and opiates really can kill people. It is also potentially inexpensive, or free if a person grows it themselves.
Physically, though, marijuana is not without drawbacks. It definitely increases appetite, which can be good in the setting of chronic illness, but can also lead to obesity. It causes men to grow breast tissue if it is used regularly, though the mechanism of this is not clear. This is primarily observational, but will probably be studied more as marijuana use becomes more common and legal. Marijuana, when smoked, does not appear to cause lung cancer or chronic obstructive lung disease, in fact it seems to be associated with increased lung capacity in regular users. There is an uncommon disease, usually of young men, called cyclic vomiting syndrome, in which patients suffer days of vomiting with intervening periods of normal gut function. This appears in many cases to be due to marijuana use, and is not limited to heavy or regular use. Marijuana is one of the many drugs that can cause pancreatitis, an inflammation of the digestive and insulin producing organ that can be painful and can eventually become chronic. It appears to be a very rare cause, though, and most people who develop this get it from alcohol abuse. There are physical symptoms of cannabis use, including conjunctival redness and increased heart rate, and there are withdrawal syndromes in regular users, including yawning, excessive sleepiness and panic attacks.
All in all, from a medical standpoint, is probably a good thing that marijuana is legalized. Patient who are presently dependent on physicians for opiate prescriptions might be able to be transitioned to marijuana, which would at least not kill them. When it becomes more practical to study marijuana's medical effects, there will be more evidence of both what it is good for and when its use should be discouraged. It is, of course, still a mind altering substance and people will need to learn how to use it responsibly. Significantly more people will probably use marijuana when they can do so without legal repercussions and physicians will probably see more issues with dependence and habitual use. This is probably an excellent time to begin to study the social and medical consequences of having a very popular and powerful chemical more generally available.