One of the original usages for medical marijuana was the treatment of something called CINV: chemotherapy-induced nausea and vomiting. This condition is a direct result of chemotherapy and is very common amongst cancer patients. In fact, the very first medical cannabis laws introduced in the 1990s were intended to protect cancer patients from prosecution. These patients had discovered that using medical marijuana helped significantly in reducing the nauseous side effects of chemotherapy and laws were put into place to make this legal. Nowadays there is a lot more than just anecdotal evidence to go on and we better understand how medical marijuana can help with nausea, and not just with CINV.
Nausea is most commonly treated with traditional drugs such as dexamethasone or antihistamines. These drugs are not always as effective as you might want and usually have a very drowsy effect on the patient, causing them to feel even worse in some cases. Since the 1980s there has been research on the nausea relieving properties of certain synthetic cannabinoids, namely nabilone and dronabinol. These drugs have since been approved by the FDA for treatment of nausea, with a special focus on CINV. In almost every trial performed these two synthetic cannabinoids outperformed their conventional counterparts, but the medical community is still very hesitant in prescribing them, a seemingly irrational decision. Many studies have been performed to alleviate concerns and the process through which these synthetic cannabinoids work is well understood: they bind to endocannabinoid receptors in the body on strategic places, and block the binding of CINV related hormones serotonin and dopamine. This combination is extremely potent and many patients have reported that their CINV related symptoms have gone away, or have at least become much less worse. Do note that this is not a claim that synthetic cannabinoids treat cancer: it is only CINV, a result of chemotherapy, that is being treated here.
Not only synthetic cannabinoids are of interest to the medical community, but natural marijuana is also looked at as an alternative way of combating CINV. Sadly research is lacking behind, with only 10 human trials ever conducted. Because of this small sample size there really is no definitive proof that medical marijuana is a good medical solution for treating CINV. Some research suggests that it might be, but nothing can be said for sure right now. There are however some studies that have looked into treating nausea as a whole using medical marijuana, so not just specifically CINV. This is even harder to study, because as opposed to CINV regular nausea does not have one singular cause: it is often a combination of multiple factors. In animals medical marijuana, and specifically cannabidiol (CBD), has however been shown to be effective in reducing nausea, so the signs are promising.
There are still no controlled human trials and so it is hard to know for sure, but it is a very real possibility that medical marijuana could be used to combat nausea. This also lines up with anecdotal evidence from patients, who have been reporting on the anti-nauseas properties of medical marijuana for decades now. marijuana does come with its own set of side effects, some of which might be less wanted than others. Some users report drowsiness and anxiety, two side effects which might not be what you are after if you are using medical marijuana to treat nausea. Some of these side effects might be less prominent when using a strain low in THC, which is the primary active ingredient in cannabis. Because many of the nausea relieving properties in cannabis seem to come from CBD it is therefore always an option to use a strain of marijuana that is low in THC and high in CBD. This way you get all the benefits with less of the side effects.