It's not a common side effect, but it's yet another addition to a long list of reasons not to see a chiropractor: They can make your eyes bleed.
In a new case study published in the American Journal of Ophthalmology Case Reports, authors from the University of Michigan report the case of a 59-year-old woman who saw three "tadpoles" in her eyes. Upon examination, the doctors didn't find any juvenile amphibians, but they did find preretinal hemorrhages; i.e., bleeding on the front side of her retinas. (The retina is the structure in the back of the eye that contains light-sensitive cells.)
Why were her eyes bleeding? Well, she went to a chiropractor who manipulated her neck using the "high-velocity, low-amplitude technique," which apparently is one of the oldest and most common chiropractic procedures. But just because they've always done it that way doesn't mean it isn't incredibly stupid. The images below depict the patient's right eye (panel A), which was hemorrhaging, and her left eye (panel C), which was not damaged. The dark spots in panel A are hemorrhages.
Though this particular side effect is probably incredibly rare, all sorts of bad things can happen after a chiropractor twists your neck. According to the article, side effects of upper spinal manipulation include "[s]troke, carotid dissection, central retinal artery occlusion, nystagmus, Wallenberg syndrome, ophthalmoplegia, Horner syndrome, loss of vision, diplopia, and ptosis."
There's also the possibility of suffering from an irreversible side effect known as death.
Chiropractic: All Risk and Little Benefit
Though the theory upon which chiropractic rests is complete pseudoscience, the practice isn't entirely worthless. A review published in 2016 concluded that chiropractic was just as effective as conventional physical therapy in treating lower back pain. The problem with chiropractic is that the practice also claims to be able to treat all sorts of medical issues, and that is bogus.
The article also casually mentions this rather appalling fact: "Americans are estimated to make more visits to CAM [complementary and alternative medicine] practitioners than primary care physicians."
Source: Yannis M.Paulus, Nicholas Belill. "Preretinal hemorrhages following chiropractor neck manipulation." American Journal of Ophthalmology Case Reports 11: 181-183. Published: September 2018. DOI: 10.1016/j.ajoc.2018.04.017