It's not ideal to smoke anything after anesthesia. People do it, but it'd be better if they didn't. For one thing, you're trying to let the anesthesia and various sedatives they give you work their way out of your system, not add more new substances. Then it's important to keep the lungs clear and smoke free after anesthesia. Weed smoke is a strong irritant. Patients are at heightened risk of developing pneumonia after they've had anesthesia, so clear, clean lungs help ward that off. Then, just like Crispi said, people's throats are already irritated from the breathing tube, and that lasts a couple of days. It's just best to take a brief smoking break from both cigs and weed.