Gaining FDA approval for a drug is a very long and costly process. As a result, many products on the market which are widely used for health purposes are not approved by the FDA for those uses. Many of these products are also safe and effective, even though they lack FDA approval.
For instance, many people take fish oil supplements to lower their cholesterol, or take calcium and vitamin D supplements for bone strength. These are not specifically approved by the FDA, but studies have shown them to be both safe and effective. Additionally, physicians often prescribe medications for uses other than their original FDA-approved uses (off-label prescribing).
It's also worth noting that FDA approval is no guarantee of safety. FDA-approved Vioxx is estimated to have caused between 26,000 and 55,000 needless deaths before it was pulled from the shelves. By contrast, there are no recorded deaths from marijuana overdose in human history.
Although the FDA has refused to study marijuana, many scientific studies have shown it to be safe and effective. For example, a randomized, double-blind, placebo-controlled clinical trial of smoked cannabis was published in the Journal of Pain in June, 2008. Patients with painful nerve injuries who smoked precisely controlled doses of marijuana experienced relief from pain which was significantly better than those who were given the placebo. The marijuana in this study was well tolerated and had minimal psychoactive effects.
Despite studies that demonstrate clear medical benefits to some patients from using marijuana, political barriers will likely continue to preclude it from being evaluated by the FDA.