Millions of people annually undergo all types of surgery with the help of these pain-relieving medications. And even though these drugs are commonplace, there are still several facts about them that may surprise you.

1. It’s possible to wake up during surgery

But it’s also extremely rare. This condition, called “anesthesia awareness,” occurs when a patient regains consciousness while they’re under general anesthesia.

According to the American Association of Nurse Anesthetists, anesthetic medications can cause unconsciousness and pain relief and (during general anesthesia-supported operations) prevent movement. If the former two effects fail, it’s possible for a patient to wake up during the surgery. And, in extremely rare cases, all three can fail, meaning the person could wake up but not be able to communicate with their doctors.

2. People who smoke may need more anesthesia than nonsmokers

Anesthesiologists have long noticed that smokers can need extra anesthetics. And now, experts are starting to confirm this:

Preliminary research presented at the 2015 European Society of Anaesthesiology meeting in Berlin found that compared with female non-smokers, women who smoked needed 33 percent more anesthesia during their operation; those exposed to secondhand smoke needed 20 percent more.

Another finding? Both of the smoking groups needed more painkiller medications after the surgery.

Smokers have irritated airways, says John Reynolds, MD, an associate professor and section head of neuroanesthesiology at Wake Forest University in Winston-Salem, North Carolina. As a result, they may need higher doses of pain medication to improve their tolerance with the breathing tubes, he explains.

3. Being obese can increase your risk of complications

It’s harder for anesthesiologists to provide the best dose of medication and deliver that medication intravenously to patients who are obese, according to the American Society of Anesthesiologists (ASA).

Plus, it’s possible for the extra weight to interfere with their breathing during the procedure. Losing weight before surgery can reduce these risks, the ASA notes.

4. You might want to try aromatherapy after anesthesia

The reason: It can quell the nausea that people commonly feel afterwards, according to a 2013 study in the journal, Anesthesia & Analgesia.

The research found that people who used ginger essential oils or a combination of ginger, spearmint, peppermint, and cardamom essential oils felt less sick after their procedure and requested fewer medications to treat their nausea. Plus, aromatherapy is noninvasive and relatively inexpensive, say the study authors.

5. Anesthesia can block your memory, too

General anesthesia isn’t just designed to keep you sedated and pain-free — it also causes amnesia, according to the NIH. That’s why, even if you do wake up during the procedure, you’re not likely to remember much about it.

6. Redheads don’t need more anesthesia than anyone else

People with red hair are likely to have a specific gene called melanocortin-1 receptor (MC1R), which was thought to decrease a person’s sensitivity to anesthetics, Dr. Harwood explains.

But that idea didn’t hold up under further scrutiny: A 2012 study in Anaesthesia and Intensive Care found that people undergoing general anesthesia didn’t find any difference in recovery times or pain between those with red hair and those with darker hair.