Can headstands be dangerous?
Not always. Aside from the more obvious risk of neck injury, the yoga headstand posture causes spikes in intraocular pressure (IOP), the pressure of fluid in the eye. A study in the American Academy of Ophthalmology in 2006 found that when yoga practitioners did headstands, their IOP doubled, on average with the increase occurring as soon as the position was assumed and the pressure returning to near-normal immediately afterward. Other papers have found that IOP increases up to fourfold.
While temporary spikes in IOP are nothing to worry about in healthy eyes, there is concern that they can, over time, progressively damage eye tissue and promote or worsen glaucoma and some other eye conditions. Thus, anyone with glaucoma or ocular hypertension should avoid headstands. To be on the safe side, people with poorly controlled hypertension should also avoid them. Older people and anyone at risk for glaucoma should get a full eye exam before doing headstands.
Some people have heartburn after doing headstands. There are no studies to confirm this, but the position could aggravate heartburn by allowing stomach acids to back up into the esophagus. People who tend to get heartburn should not even lie down after meals, let alone do headstands.