THC is the most well known cannabinoid present in cannabis. It is the main psychoactive compound, responsible for the high experienced by recreational users. It is also the most established medicinal compound in the cannabis plant.
Like other cannabinoids, THC is so non toxic that an achievable toxic dose has not been established with estimated quantities required for a lethal dose leading to the conclusion that a lethal overdose of THC, or cannabinoids in general, is not physically possible to achieve.
THC is not produced directly by the plant but is actually created by a process called decarboxylation where the raw THCA, that is produced by the plant, converts to THC through a combination of heat and time.
The medicinal properties of THC are quite profound and the safety profile unmatched by any other therapeutic substance or drug, though the psychoactive effects do limit doses that are comfortably tolerated.
THC has shown potent anti cancer, anti spasmodic, anti emetic (anti nausea), anti oxidant, analgesic (pain relieving), appetite stimulating, neuro-protective and immune modulating effects. It has also demonstrated unmatched efficacy for treating glaucoma, many of the symptoms of AIDS and PTSD, tremors associated with Parkinson's disease and similar conditions. It has also been demonstrated that THC can prevent the plague build up that causes Alzheimer's disease and a potential to actually treat, even reverse, Alzheimer's disease is currently being investigated.
The evidence for the anti cancer effects of THC date back to a 1974 University of Virginia study.
http://www.cancer.gov/about-cancer/treatment/cam/hp/cannabis-pdq#section/all
Though numerous other studies have supported the initial findings, with significant anti cancer effects demonstrated in both vitro (lab) and vivo (animal models) studies, the legal restrictions have severely hindered research on human patients.
However, in addition to the numerous anecdotal reports of cannabis, primarily high THC extracts, successfully treating serious late stage cancers, there is also some clinical data related to human patients and a small number of case studies.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3901602/
http://www.collective-evolution.com/2013/08/23/20-medical-studies-that-prove-cannabis-can-cure-cancer/
http://www.cureyourowncancer.org/dr-william-courtney-calls-child-a-miracle-baby.html
The anti cancer effects of cannabinoids, THC in particular, appear to be profound enough to negate any carcinogenic properties of the smoke, to the point that smoking cannabis is not considered to increase the risk of developing cancer. This was demonstrated by a large study conducted by Dr Tashkin on cannabis smokers compared to non smokers and tobacco smokers, with cannabis smokers demonstrating less likelihood of developing head, neck and lung cancers. These results allude to a protective effect against cannabis, even from smoking cannabis.
http://www.washingtonpost.com/wp-dyn/content/article/2006/05/25/AR2006052501729.html
The evidence for THC as a treatment for Glaucoma also dates back many years with Robert Randall the first person to successfully sue the US Federal Government, to the point that their own courts ordered them to supply Mr Randall with medical cannabis for the rest of his natural life. Despite being told by doctors that he would be blind by the time he was 30, Mr Randall never lost his sight and continued to smoke federally supplied cannabis to treat his Glaucoma until his death at the age of 53 from AIDS related complications.
http://www.nytimes.com/2001/06/08/us/robert-randall-53-sued-for-marijuana.html
Further studies have confirmed that THC does indeed reduce intraocular pressure to prevent glaucoma progression from causing blindness.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772142/
http://journals.lww.com/glaucomajournal/Abstract/2006/10000/Effect_of_Sublingual_Application_of_Cannabinoids.1.aspx
In addition to the numerous established palliative benefits for AIDS patients, like appetite stimulation and pain relief, evidence is emerging that THC may have potential to actually treat the disease.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0033961
http://www.jleukbio.org/content/93/5/801.abstract
http://www.wired.co.uk/news/archive/2013-05/1/cannabis-hiv-drug
Irritable Bowl Syndrome and related conditions
http://www.ncbi.nlm.nih.gov/pubmed/23769745
In addition to the evidence supporting the potential for THC to prevent Alzheimer's Disease, there is increasing evidence that it may even be able to treat the condition.
http://www.ncbi.nlm.nih.gov/pubmed/23515018
http://www.naturalnews.com/046219_cannabis_alzheimers_disease_prevention.html
In addition to the numerous anecdotal cases that show that THC rich cannabis can be very beneficial for sufferers of PTSD, studies are confirming these benefits.
http://newmedia-eng.haifa.ac.il/?p=5527
http://www.maps.org/maps-media/media/5077-marijuana-use-may-improve-ptsd-study-finds
http://www.npr.org/sections/health-shots/2013/12/23/256610483/could-pot-help-veterans-with-ptsd-brain-scientists-say-maybe
THC has shown unsurprising efficacy at treating Anorexia and wasting disorders, largely thanks to the well established appetite stimulation properties
http://www.ncbi.nlm.nih.gov/pubmed/21412227
THC also shows tremendous benefit for Parkinson's Disease, with both anecdotal and preclinical evidence gaining attention
http://www.medicaljane.com/2014/11/14/parkinsons-disease-and-medical-marijuana/
The benefits of high THC cannabis reported by real life MS sufferers is gaining more scientific support
http://brain.oxfordjournals.org/content/126/10/2191
http://www.karger.com/Article/Abstract/112901
http://msj.sagepub.com/content/10/4/417.short
Along with CBD (with both compounds enhancing the efficacy of the other) THC has demonstrated the ability to aid bone growth and repair.
http://www.ncbi.nlm.nih.gov/pubmed/25801536
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3165946/
THC also shows great potential for treating Depression, which makes it considerably useful for sufferers of depression as well as sufferers of other conditions and diseases where depression can be an issue
http://www.sciencedirect.com/science/article/pii/S0091305710000730