After surviving the wars in Iraq and Afghanistan, former soldiers say they are now getting hit by debilitating headaches several times a week.

When the headaches hit, veteran Jody Fuller sees black spots. His peripheral vision disappears. And he knows, he has to stop everything.

“When I have terrible headaches, that’s when I have to go somewhere, shut the curtains, put the pillow over my head. The light is not a good thing. I’m sensitive to light,” said Fuller, who served three tours in Iraq.

Fuller is not alone.

Headaches are one of the many lingering effects for thousands of U.S. soldiers who fought in Iraq and Afghanistan the past 12 years.

The headaches can be crippling for these veterans. Some report having three to four migraines a week. The pain is so intense, they cannot drive, work, or carry on with a typical day.

Other veterans have reported chronic headaches that follow them throughout the day, flaring in intensity. In some cases, headache sufferers lay in the dark for days, isolated, avoiding the light.

Fuller told Healthline he’s “really blessed” despite his headaches. They occur four days a week as opposed to “getting it every single day.”

“Mine really pale in comparison,” he said.

But the Army reservist admits the headaches can be disruptive, forcing him to stop and drop, waiting for the pain to subside.

“Sometimes, I wake up when the headache is gone, but my head still physically hurts. The brain part is gone, but the physical skull hurts,” Fuller said.

Read More: Vietnam Veterans Still Have PTSD 40 Years After the War »

More than 6,800 U.S. military personnel have been killed in the Iraq and Afghanistan wars.

In addition, more than 52,000 U.S. personnel have been wounded and another 320,000 have suffered traumatic brain injuries, according to the Wounded Warrior Project.

Many of the veterans’ headaches stem from these traumatic brain injuries, such as concussions and head trauma after exposure to blasts, falls, or car accidents suffered in Iraq or Afghanistan.

A 2011 study in the journal Headache found that 74 percent of veterans with mild traumatic brain injuries suffered headaches within a month of the incident and that 33 percent went on to have persistent headaches.

Vets have described pain in multiple places, likening it to lightning bolts striking the sides of their head, pain coursing through their eyes and neck, and dizziness. The types of headaches vary, as well as thecause andseverity.

“If you dig into military headache, not all are the same,” said Don McGeary, Ph.D., an assistant professor and researcher at the University of Texas Health Science Center San Antonio, who studies military chronic pain management. “These are migraines throbbing with sensitivity to light, tension headaches, band tightening around the head. You could have cluster headaches, chronic daily headaches. Headaches phenotype in different ways.”

Much like the brain, headaches are also not well understood. And they could emerge years after the soldiers leave a combat zone.

Fuller’s last deployment to Iraq was in 2011, but he still has headaches. And the root of the pain still hasn’t been pinpointed.

“I went to a neurologist and he ran several tests,” Fuller said. “Everything came back normal, and that’s part of the problem. There’s a lot of folks who seek medical treatment and the docs just can’t find anything.”

Read More: Female Marine Breaks Silence on PTSD Struggle »

Fuller didn’t have a blast-associated head injury. But military personnel who have not suffered concussions or explosive-related trauma are also prone to persistent headaches.

“It’s not just blast injuries or mechanical injuries or getting knocked around. It’s also getting exposed to fine particulate [in the air] that can cause problems,” McGeary said.

Fuller spent a lot of time near burn pits, where the military disposes of its trash and human waste.

“There are things we had to dispose of and burn, no telling what it could’ve been,” he said. “I know, afterward, I was worried. The burn pits and just breathing the sand, it’s just not good for human consumption.”

The U.S. Department of Veterans Affairs acknowledges that smoke from burning trash pits can cause symptoms such as headaches, nausea, and other irritations, and that soldiers can experience these long after leaving the zone.

But a National Academy of Sciences Institute of Medicine study released in 2011 indicated that there are no long-term adverse health effects from exposure to the burn pits and that such pollutant levels may not be higher than other sites. But the topic also remains riddled with questions.

Other major factors contributing to headaches are stress and poor sleep.

“You sleep poorly through an extended period of time. That can contribute to headache and the significant stress of the job. When you have extended stressors, that can promote vulnerability to headache,” McGeary said.

In combination with poor sleep, traumatic brain injuries, stress, chemical exposures, and PTSD, headaches in veterans remain complex.

Read More: Children in Military Families More Likely to Have Problems »

The U.S. government has shown increased interest in non-pharmaceutical treatments for headaches.

“Military service members in general don’t like medication for pain,” McGeary said. “One of the reasons is that side effects tend to be contrary to military service when you have a sedating drug. People with chronic pain have a tolerate/hate relationship with pain meds because it could be prohibitive of activity in general.”

Research has examined non-medication treatment options to address stress, depression, sleep health, and PTSD. Instead of relying on pills to deal with the pain, researchers are seeking to address the triggers that could start the headaches.

These treatments include massage, relaxation skills, stress management, and sleep hygiene counseling.

Fuller says he uses Tylenol PM to help get through the headaches.

With the wars in Iraq and Afghanistan fading from headlines, Fuller laments, “People forget about the veterans.”

“We need to do a better job talking about these issues. A lot of us share the same conditions,” he said.