It's a fair question, usually asked of a bottle that has waited at the back of a cabinet since the last celebration. The short answer is no, tequila doesn't spoil the way wine or milk does. It's bottled at between 35 and 55 percent alcohol, which leaves nothing for spoilage to work with. But a spirit that can't go bad can still lose itself, and that's the part worth understanding.
Tequila doesn't age in the bottle. The time a reposado or añejo spends in oak stops the day it's bottled, so a blanco kept for ten years is still a blanco. What changes after opening is the air. Every pour leaves a little more room in the bottle, and oxygen slowly softens the brighter notes first: the cooked agave, the citrus peel, the pepper. Over months that change is barely noticeable. Over a year or two, in a bottle that's mostly air, the spirit can taste flatter and quieter than it should.
Light and heat speed this up. Keep tequila out of direct sun and away from the stove, the window ledge and the top of the fridge. In an Indian summer, a cool, dark cupboard is kinder than any display shelf. Store the bottle upright, too. Unlike wine, a strong spirit resting against a natural cork will slowly wear it down, and a crumbling cork lets in more air.
When a good bottle is down to its last third, it's worth moving it into a smaller, clean glass bottle that closes tightly. Less air means less change. Unopened, tequila will keep for years if it's stored the same way. The only real enemies are a poor seal and a warm room.
So the old bottle is almost certainly safe to pour. Whether it still tastes the way it was made is another question, and an easy one to answer: pour a small measure, let it rest, and taste it before you decide. Something made with patience should be kept with a little care.