Minor things are local pain, maybe bruising and a drop of blood could come out. Even these are rare when done by a professional. More serious things would be nerve pain or a patient passing out. Again, those are rare and usually the passing out comes from the patient not eating that day, not resting well or because of their fears (similar to someone passing out when getting blood drawn is more from the fear than the actual draw).
Very serious problems, such as pneumothorax (lung collapse) or piercing a vital organ, etc are so rare that I only know of one person doing it in my 20 years of practice. This type of occurrence happens by people who have minimal training or don't care about what they are doing
Licensed Acupuncturists are trained to avoid such occurrences more vigorously than those who take the weekend courses to add to their scope (see MD/DC/PT note above).
There really is no danger of them needling "wrong points" and you won't have any real adverse effects from the treatment itself, it isn't like a pharmaceutical that would poison you or have an overdose limit. You might feel more calm and sedated as your body regulates, many patients feel a better sense of well being and that can seem weird, but it isn't harmful.
Any fear you have, be sure to just ask your practitioner. I promise you it is safer than the meds you are already taking.
Anticoagulants do not cause more bruises or bleeding in acupuncture treatment. Interesting, right?
During the last ten years in my 35 years of practice, more and more people ask for non-pharmaceutical therapies to cut down drugs usage. Make any sense now?
Hope this helps ease your worries.
-An occasional bruise or tender spot
-A red spot (histamine response) where some of the needles were (painless and don't itch)
-Mid- and/or post-acupuncture itching (feel the qi moving!)
It's also normal for pain levels to fluctuate a bit at first, so it may seem like there is a "flare-up," but generally, it's considered normal and a sign of change, and change is good.
As far as what if acupuncture is performed incorrectly (very rare if performed by a licensed acupuncturist), there are some rare complications that could happen such as pneumothorax (punctured lung). This is very rare due to both the small size of modern needles and the anatomy understanding of the acupuncturist. Though I've heard of a case in Canada and one in the UK (both times the acupuncture was performed by a massage therapist and not an acupuncturist). I heard about a case in Korea where a needle was inadvertently left in and worked it's way into the lung and was removed with surgery. I was always suspicious of that story as such an event is unlikely, but I suppose it would be possible. The other complication that could happen is called a "stuck needle" which is where the needle gets stuck in the fascia. This happens for two reasons: 1. The needle is rotated during treatment (this is normal) and somehow gets caught in the fascia which doesn't seem to want to let go or 2. The body simply wants to hold onto it little longer. This resolves by leaving the needle in place a few more minutes or inserting another needle next to it to release tense fascia or muscle. I've only ever had this happen once in my career and I was able to get it to release by rotating the need the opposite direction and gently tapping the skin near the needle.
Fainting is another rare side effect. I've had this happen four times in 7.5 years and 3x the person had diabetes, was nervous about treatment and hadn't eaten for several hours prior to treatment. I realize these things are all alarming, but I cannot stress how rare and unlikely there are and millions of treatments are given around the world each year without any adverse effects.
With that statement out of the way, generally speaking acupuncture is very safe. In increasing order of severity (and decreasing order of probability), here's what could possibly go wrong:
1. Local bruising/itching at the needle site. Itch is probably the most common adverse event and usually resolves in an hour or two. Bruising is probably the second most common and usually resolves in a couple of days.
2. Sharp pain around the needle site. Typically this means we're a little close to a nerve or capillary bed. Often times, repositioning the needle will make this pain go away. It's important to provide feedback to your provider (and important for providers to solicit this feedback). If you experience sharp pain on needle insertion that does not go away after a couple seconds, tell your acupuncturist and have them either move or remove that needle.
3. Sharp pain that does not subside when the needle is repositioned or removed. This is a little more serious and may mean the needle has nicked a larger nerve. Sometimes this can take a couple days to clear up.
4. Organ puncture, including tension pneumothorax. Organ puncture only occurs when needling over the abdomen (front or back). Pneumothorax only occurs when needling over the lungs (front or back). Of the two, pneumothorax is the more serious. Basically what has happened is the needle was pushed too deep and invaded the pleural/lung space allowing air from the outside to get in. This is a potentially life-threatening condition and requires immediate ER attention. Symptoms of a pneumothorax are: chest pain/tightness, painful cough, shortness of breath. If you experience any of these symptoms after receiving acupuncture over the chest or at the top part of the shoulder, go directly to the ER.
There is a 5th issue which very, very rarely pops up and that is infection. Almost all US acupuncturists use single-use disposable needles. This mean the needle is used once and then thrown away, reducing the possibility of cross-contamination between patients. Additionally, NCCAOM board certified acupuncturists are required to obtain a Clean Needle Technique (CNT) certification prior to receiving their board certification and state licensure. CNT spells out all the requirements for maintaining a clean space for the practice of acupuncture. Receiving this certification consists of being trained in school, attending a seminar, taking a written test and then taking a practical exam where an exam proctor looks over your shoulder while you're inserting and removing needles to ensure you're doing so in a safe and clean manner. Since the implementation of these guidelines and certifications, there have been very few instances of infection due to cross-contamination.
In my experience, the first adverse event happens quite a lot. Itch is probably the single biggest piece of feedback I receive from patients after a treatment.
The second event happens a fair amount, but getting feedback and repositioning or removing needles takes care of the problem and I've never had someone not schedule or re-schedule with me because the needles were too painful.
I've never had the 3rd, 4th or 5th adverse events occur while providing a treatment. To be honest, I've never heard of a board certified acupuncturist causing the 3rd or 4th issue. The only acupuncture cross-contamination case I'm familiar with comes from Canada in the 1980's.
Which brings me to my last point. Throughout this answer I've referenced NCCAOM board certified acupuncturists. These are folks who attended at least 3 years of Chinese medical school, sat for multiple board exams, acquired CNT certification and are state licensed as providers of Chinese medicine. Unfortunately, these aren't the only folks who offer treatment using solid acupuncture needles. There are other providers in the system who offer acupuncture-like services with a fraction the training and, in my opinion, you are rolling the dice with these providers.
To find an NCCAOM board certified acupuncturist close to you, go to NCCAOM.org and use their "Find a Practitioner" look up. Making sure your provider carries an NCCAOM board certification, CNT certification and state practice license will greatly minimize the probability you'll experience a serious adverse event.