Antiemetic (anti-nausea) use
Early on, THC had been shown to be effective for some patients who suffered nausea from cancer chemotherapy treatments. However, the narrow window between the anti-emetic dose and that which caused unwanted psychic effects made THC difficult to use.3 In some studies, negative side effects occurred in up to 81% of patients.4 In one of the few studies using smoked marijuana, 20% of patients dropped out of the study, while another 22% reported no relief of nausea symptoms.5 The advent of serotonin 5-HT3 receptor antagonists as new and more powerful anti-emetic drugs that were free of unwanted psychic effects has made cannabinoid use less attractive. For this reason, physicians virtually never prescribe marijuana or THC as an antiemetic for use by chemotherapy patients.6
Multiple sclerosis
Studies have shown that cannabis can relieve muscle pain and spasticity in patients suffering from multiple sclerosis7 and can control tremors in multiple sclerosis animal models.8 However, a study in ten patients with spastic multiple sclerosis showed that smoking marijuana further impaired posture and balance in those patients.9 In addition, MS patients who used marijuana had a greater number of psychiatric diagnoses and a slower mean performance time on standard neurological tests.10 Some randomized, double-blind, placebo-controlled, parallel group crossover trials have found no significant improvement of MS symptoms during cannabis plant extract use. However, in some trials patients did show an increase in aggressive behavior and paranoiac tendencies in a standard psychological test.11 Another placebo-controlled study, examining the effect of a cannabis extract on spasticity in MS, found a positive partial relief of symptoms in 40% of patients.12 A 10-week, placebo-controlled study of MS patients found that 42% withdrew due to lack of efficacy, adverse events and other reasons. Patients reported 292 unwanted effects, of which 251 were mild to moderate, including oral pain, dizziness, diarrhea, nausea. Three patients suffered five serious adverse events, including two seizures, one fall, one aspiration pneumonia, one gastroenteritis. Four patients had first-ever seizures. A minority of patients received some relief of symptoms.13 So, overall, studies show that a minority of multiple sclerosis patients can receive some symptom relief through the use of marijuana extracts or THC, although a significant percentage of patients suffer unwanted adverse effects.
http://www.godandscience.org/doctrine/medical_marijuana_review.html
As with most substances there are positive effects and negative effects.