Obstructive sleep apnea (OSA) is a severe type of snoring causing people to choke in their sleep. It affects millions of Americans, causing many health problems. For example, patients with OSA often feel very sleepy and are at risk of falling asleep while driving. OSA also causes elevated blood pressure increasing the risk for heart attacks and strokes. Patients with OSA are often treated with a face-mask that helps them breath at night but can be difficult to tolerate. In fact, about half the patients eventually stop using this mask. Because there are few other treatments (and no drug therapy), many OSA patients are still untreated. Of note, especially young adults (i.e. 18 to 50 years old) benefit from treating their OSA, but they are also less likely to use the mask. Acetazolamide (a mild diuretic drug) has been used for over 50 years to treat many different conditions and is well tolerated. Recent data suggest, that acetazolamide may help OSA patients to not choke in their sleep and lower their blood pressure. Especially young adults with OSA are likely to respond well to this drug. Further, its low cost (66¢/day) and once- daily dosing may be particularly attractive for young OSA patients unable or unwilling to wear a mask each night. But previous studies had many limitations and did not focus on young adults. The goal of this study is to test if acetazolamide can improve sleep apnea and cardiovascular health in young adults with OSA (18-50 years old), and how it does that. Thus, we will treat 46 young OSA patients with acetazolamide or placebo for 2 weeks each. The order in which participants receive the drug or placebo will be randomized. At the end of each 2 week period we will assess OSA severity and cardiovascular health. Thus, this study will help assess acetazolamide's potential value for OSA treatment, and may also help to identify patients who are most likely to respond to acetazolamide (including select individuals \>50 years of age). Ultimately, this work promises a drug therapy option for millions of OSA patients who are unable to tolerate current treatments.