There are no carbohydrates in tequila when it is drank straight without any mixers. However, when it is used in a mixed drink, the carbohydrate count is based on the mixers used and is typically quite substantial.
When you are counting carbohydrates for weight loss or a health condition, you should count every ingredient in the consumable. You can drink tequila and not worry about carbohydrates; however, since it is typically an ingredient in a drink, you must count every ingredient. The majority of mixers are sugar laden and thus full of carbohydrates. Additionally, if you are counting carbs, be aware that alcohol tends to increase your appetite and thus your craving for carbs.