Inclusion Criteria:
* Gulf War Veterans of the U.S. military deployed on military orders to the Persian Gulf Region between August 2, 1990, and December 31, 1991
* Diagnosed with GWI according to the CDC and modified Kansas criteria as recommended by the Institute of Medicine (IOM), that is, endorse multiple or moderate-to-severe symptoms, with symptom onset during or after deployment to the Persian Gulf region in 1990-1991, persisting for six months or longer, in at least 3 of 6 domains:
1. fatigue/sleep disturbances
2. neurological/cognitive/mood symptoms
3. somatic pain
4. gastrointestinal problems
5. respiratory symptoms
6. skin symptoms
* Self-reported cognitive dysfunction based on a T-score of 40 or less on the PROMIS v2.0 Cognitive Function 8a short form
* Be able to:
* provide written consent and be able to communicate with the research team in verbal and written English
* attend the two in-person study encounters
* have reliable telephone service for the eight weekly telephone encounters
Exclusion Criteria:
* Diagnosed by a physician with any chronic condition that may explain their profile of symptoms or prevent their ability to accurately report them including:
* chronic autoimmune conditions
* systemic inflammatory conditions
* cancer not in remission at least 5 years
* congestive heart failure
* anemia
* multiple sclerosis
* amyotrophic lateral sclerosis (ALS)
* poorly controlled diabetes
* post-chemo or radiation syndromes
* sickle cell anemia
* symptomatic Coronary Artery Disease (CAD)
* chronic liver disease
* chemical insufficiency
* morbid obesity (body mass index (BMI) \>= 40)
* human immunodeficiency virus (HIV)
* alcohol/substance use disorder/stimulant/opioid/other depressant misuse in the past year
* major mental health condition (e.g., psychosis, suicidal ideations, major depressive disorder) that interferes with their ability to accurately report symptoms
* hospitalized or undergoing invasive procedures in the past 12 months due to exacerbations of any chronic conditions (such as diabetes, coronary artery disease, hypertension, or emphysema)
* elevated liver enzymes (2.5 times upper limit of normal) at baseline visit
* estimated glomerular filtration rate less than 60 ml/min/1.73 sqm at baseline visit
* hemoglobin less than 10 g/L at baseline visit
* evidence of poorly controlled chronic conditions listed above, or others that may mimic GWI as per the PI, either by self-report, Veterans Health Administration (VHA) electronic health record information review, laboratory testing or physical examination
* Changes in medications for chronic conditions in the 3 months preceding enrollment (based on self-report)
* Suicidal ideation based on their responses on the Columbia Suicide Risk Inventory
* Prescribed or taking Montelukast in the past 6 months for any reason
* Taking 2 or more medications with moderate interactions with Montelukast
* Pregnancy or intention to become pregnant
* Active homicidal ideation
* COVID-19 illness (confirmed or suspected) without recovery to pre-COVID health status
