In recent years yoga has saturated the fitness market. Endless urban lofts house whitewashed specialist centres, yoga retreats are among the most popular holidays, and you would be hard pressed to find a gym in the land that doesn't boast at least one type of yoga class, be it classic Hatha poses, Bikram's sweaty sessions in a heated studio or dynamic Ashtanga yoga, also known as "power yoga".

There are signs, however, that this bubble may be about to burst. A recent article in Time magazine reported that orthopaedic surgeons, physical therapists and chiropractors in America are increasingly dealing with the fallout of yoga practice gone awry and a rash of injuries are blamed on the fact that as teaching and practice of yoga spreads, so do bad examples of both. The magazine also argues that as a form of exercise, yoga is largely ineffective, bestowing little in the way of a cardiovascular workout or weight-loss benefits.

Robin Shepherd, acting chairman of the General Osteopathic Council, agrees that yoga has its risks: "People get lulled into a false sense of security because yoga has the image of being a very low-impact activity. I see patients who are injured as a result of yoga."

Typically, for example, people with painful backs will go to a yoga class thinking it will solve their problem. "But lots of people have relatively hypermobile areas of their spine, which they find very easy to stretch, so the yoga doesn't touch the stiffer bits, and yet they throw themselves into the postures thinking it's good for them," says Shepherd. At best this is merely ineffective; at worst it can be damaging.

As well as a poor understanding of the practice itself, badly trained, inexperienced or overzealous teachers can also cause problems, as Shami Choudhry, who works in marketing in Manchester, discovered. "I was at an Ashtanga yoga class and in a posture where you really twist your spine. My teacher came to adjust me in the pose and really pushed me into it. It felt really uncomfortable at the time and it got worse afterwards. It was so painful that I went to the doctor and he told me that I'd bruised a rib."

Though an experienced yogi, Choudhry was uncertain about questioning her teacher. "When you're in the middle of a class it's really difficult to say, 'Ow, that hurts,'" she says. "My teacher was quite a forceful character and the assumption was he knew what he was doing."

Not all yoga teachers are created equal. There is no legislation that dictates standards or training for yoga teachers and, with spiralling demand for classes, there is nothing to say that the hour-long session in your local gym is supervised by someone with the right credentials. "It's certainly possible that the spread of yoga means that there are some bad teachers out there," says Helen Smith, chairwoman of the British Wheel of Yoga.

But, equally, she points out that some very good teachers hold classes in gyms and health centres and there is a very simple way to find out about your teacher: ask them. "Any yoga teacher should be proud to tell you where they trained," Smith says. Key elements to inquire about are how long they have been training and their understanding of anatomy and physiology. They should also always ask about pre-existing injuries and be able to offer thorough advice on what's suitable for you. "British Wheel of Yoga teachers train for three years and we usually require them to have done quite a lot of study beforehand," says Smith. "Safety is paramount all the way through it. Teachers are trained to watch students carefully and give individual help."

But, above all, yoga students can help themselves by taking responsibility for their own practice. Jonathan Sattin, managing director of the Triyoga centres in London, says: "It's not like getting on a treadmill. It requires mindfulness and understanding of your own body. You have to learn to recognise pain that isn't good and stop if you need to. It's not a competitive practice and forcing yourself into positions won't do you any good."

Shepherd suggests that you need to be aware of the areas of strength and weakness in your body so you can inform your yoga teacher; and to take a measured approach to yoga practice, as with any exercise."People do binge on these things," he says. "Small, regular amounts of any type of exercise are much better than overdoing it."

Amanda James, a Hatha yoga teacher at Yogahome in north London, agrees. "There is an injury risk in anything," she points out. "And people forget that yoga is a lifetime practice. More than that, it's a spiritual practice. The physical benefits come with it, but if you're only interested in getting fit as quickly as possible, go to the gym."

And what about the suggestion that yoga isn't a "proper" workout? "Try telling that to someone who has just done a two-hour Ashtanga class," says Sattin. "But that's really missing the point of the practice. The benefits of yoga are about total wellbeing, not well-developed thigh muscles."

As larger numbers of people experiment with yoga, it stands to reason that there will be more people around who don't enjoy it or have a rogue teacher. That's just statistics. But if you keep in mind that it can be both physically and mentally demanding, and take responsibility for your own safety, you shouldn't face any greater risk than you would from any other physical activity. That's just common sense. As James says: "It's only if you jump headlong into something without knowing what it's about that it's going to come back and bite you".

How to practise yoga safely

Check the training, qualifications and experience of your teacher.

If you're a beginner, take a beginner's class, even if you think you're fitter or more flexible than the average newbie.

Always inform your yoga teacher when you first meet them about any injuries, niggles or worries you have.

Don't push yourself unnecessarily. The practice is about progression at your own pace, not keeping up with the person next to you.

Ask questions if you're unsure of a move or posture. Subtle changes in alignment can make a big difference.

Don't be afraid to tell your teacher if something is uncomfortable, or to come out of a posture early if it's becoming too much.