Sativa strains of cannabis, as opposed to Indica, typically have a higher THC content. The THC is secreted by the resinous glands of the plant, located on the trichomes of the flowering blooms of the plant (the buds). As such, THC itself has no odor - rather the odor of cannabis comes from the natural terpenes present in the plant. THC is responsible for direct stimulation of the endocannabinoid system in the body, and activates the same receptors as the body's own endogenous cannabinoid Anadamide.
The chief psychoactive effect of THC varies greatly between individuals, but there is certain commonality of the effect. Main effects of THC ingestion are related to having an effect on memory, pleasure, coordination, thinking, time perception, sleeping and eating patterns (the munchies), as well as a host of others. The effect is generally dose dependant, and this depends on concentration, ingestion technique, absorption, as well as surroundings and your physiologic state.
Medicinally, major effects are analgesia (pain reduction), anti-spasmotic, muscle relaxant, anti-inflammatory, appetite stimulant, and anti-emetic. THC is a bronchodilator useful in asthma, especially when combined with the terpene a-pinene. We can consider these in a little more detail, and keep in mind their are other effects not as prominent, and some quite likely yet to be discovered. For pain reduction, cannabis works best for neuropathic pain (pain caused by the neurons themselves) and not proprioceptive pain like getting a tooth pulled. As an anti-spasmotic in can be used in treatment of motor oriented diseases, disorders of muscle spasms, and perhaps epilepsy, Parkinsons, and spastic colon. It is somewhat useful as a direct muscle relaxant, but typically a lesser effect than pharmaceuticals ie. valium, xanax, flexeril, etc.. It is also an anti-inflammatory agent, as well as an anti-oxidant. As an appetite stimulant it possibly ranks as one of the best, if not the best! Note: some strains being produced today actually are bred to decrease appetite. These strains are high in THCV - tetrahydrocannabivarin and inhibit food intake! For patients with nausea and vomiting, for example chemo patients, it is a strong anti-emetic agent comparable or better than most pharmaceuticals. This is particularly true for delayed nausea which can occur a day or two after your chemotherapy infusion. There are other effects as well not discussed here.
The effects after ingestion depend on the vehicle used to ingest. For smoking or vaping, the effects are felt within minutes and in some cases (ie bong hits, dabbing, etc..) just a few seconds - for edibles it can take an hour or two, but most people report about 30-60 minutes for onset. The duration once again dependent on other factors, but generally it will be 2-4 hours before it starts to wear off, quite a bit longer for edibles, which have a prolonged effect. Sometimes there are initial overwhelming effects and increase feeling of unease or anxiety, and this usually dissipates if untreated in about 10 -15 minutes. Sedation effects are quite common but usually occur in the later stages after ingestion or smoking, and in fact THC can be a good sleep inducer and help with insomnia or sleep disorders and PTSD. THC when metabolizied breaks down partially to CBN - cannabinol - which is a sleep inducing sedative.
Adverse or negative side effects do exist but often require no intervention other than supportive measures, and there is really no lethal dose as it does not cause respiratory depression like narcotics and fentanyl. It is possible it can aggravate schizophrenia, and must be used carefully in cases of mental disease or illness. Not recommended for pregnant women, or for individuals under the age of 18. Adverse Side Effects
Ongoing research will undoubtdly uncover new uses and medicinal therapies for THC. New research is finally being allowed and accepted after years of suppresion. It is also important to note that THC by itself (ie marinol), has limited effects. It is most therapeutic in combination with other ingredients in the plant, mainly the cannabinoids and terpenes for a synergistic effects commonly called the entourage effect. Importantly, THC is also antagonized by CBD (cannabidiol) which is perhaps the second most important cannabinoid, and a main contributor to the plant for it's medicinal properties. Interestingly, THC breaksdown with age to CBN (cannabinol) which has quite a sedative and sleep inducing effect, and lacks much psychoactivity. This is why old or stale cannabis lacks proper stimulatory effect and is sleep inducing.