The health benefits of alfalfa are largely unsupported scientifically, although there is plenty of anecdotal enthusiasm for it. Alfalfa is highly nutritious, containing approximately 16% protein, and 8% of minerals like calcium, iron, potassium, and magnesium. It also contains small amounts of the vitamins A, D, E, and K.
Alfalfa is believed to support the immune system, help in normalizing blood clotting, assist with the absorption of nutrients, alleviate hot flushes, cleanse the blood, and promote heart health.
New research also found that alfalfa sprouts and other sprouted vegetables (like broccoli and clover sprouts), may help protect new cells from DNA damage. In the trial conducted on human cells in vitro, researchers at the University of Ulster's School of Biomedical Sciences found that whilst sprouts didn't improve the health of already damaged DNA, it did protect cells from new damage. DNA damage is a biochemical indicator of cancer.
However, there are a few things to consider before adding a lot of sprouts to your diet, or supplementing with alfalfa.
Sprouts can harbor bacteria that leads to food poisoning. This is particularly of issue for children, the elderly, and people with immune related illnesses, like Aids. The problem can be eliminated if sprouts are cooked, but then you don't get the benefit of all the enzymes in them, and some vitamins may be lost. There was an outbreak of salmonella in 2002 in California, Arizona, New Mexico, and Colorado, and the FDA recommends that sprouts be cooked first.
And in 1995, alfalfa sprouts were associated with an outbreak of salmonella in Chicago. 17 people experienced symptoms ranging from diarrhea, vomiting, fever, and headaches, and 4 people had to be hospitalized. Many organic growers, however, feel their seeds are safe, and the risk is exaggerated. Washing sprouts well can minimize the risk of bacterial poisoning like salmonella. Thorough washing also removes mold, another possible problem.
One potential issue intrinsic to alfalfa is that it has been associated with systemic lupus erythematosus (SLE) in animal studies (as opposed to human trials or cases). Alfalfa supplements have been associated with the activation of dormant SLE as well. SLE is an autoimmune disease, which can damage the kidneys and other organs.
Alfalfa can also interfere with the way the body uses vitamin E, and has been linked to abnormalities in the blood. These possible issues make taking alfalfa supplements long term a bit risky.

References:
1. nutraingredients.com/news/ng.asp?id=52995
2. nutraingredients.com/news/ng.asp?id=34189
3. Dr Kenneth Cooper, Advanced Nutritional Therapies