网址在这里
http://nccam.nih.gov/health/yoga/introduction.htm

是在 NIH 的 National Center for Complementary and Alternative Medicine下面。

关于副作用和危险，它有如下说法：
Side Effects and Risks

Yoga is generally low-impact and safe for healthy people when practiced appropriately under the guidance of a well-trained instructor.
Overall, those who practice yoga have a low rate of side effects, and the risk of serious injury from yoga is quite low. However, certain types of stroke as well as pain from nerve damage are among the rare possible side effects of practicing yoga.
Women who are pregnant and people with certain medical conditions, such as high blood pressure, glaucoma (a condition in which fluid pressure within the eye slowly increases and can damage the eye’s optic nerve), and sciatica (pain, weakness, numbing, or tingling that can extend from the lower back to the calf, foot, or even the toes), should modify or avoid some yoga poses.

看这个说法，瑜伽还是个比较安全的运动。我想，如果训练有度，又有有执照的专业人员指导，应该没有多么危险。还有最好别作什么hot yoga 之类。

而且，据说是有减轻背痛，降低心跳血压，减少精神压力的健康益处。

Key Points

Recent studies in people with chronic low-back pain suggest that a carefully adapted set of yoga poses can help reduce pain and improve function (the ability to walk and move). Studies also suggest that practicing yoga (as well as other forms of regular exercise) might have other health benefits such as reducing heart rate and blood pressure, and may also help relieve anxiety and depression. Other research suggests yoga is not helpful for asthma, and studies looking at yoga and arthritis have had mixed results.
People with high blood pressure, glaucoma, or sciatica, and women who are pregnant should modify or avoid some yoga poses.
Ask a trusted source (such as a health care provider or local hospital) to recommend a yoga practitioner. Contact professional organizations for the names of practitioners who have completed an acceptable training program.
Tell all your health care providers about any complementary health practices you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.