There are physical and mental risks to hypnosis. They are unlikely, but they do exist. The only book I’ve found specifically on hypnotic risks is Hypnotic Complications, but by its nature it tends to focus on hypnotherapy and stage hypnosis cases where there is minimal communication. This is a list of what you’re likely to run into as a newbie working in close communication with your partner.
By far the most common risk in hypnosis is purely physical. Subjects in a hypnotic trance will lean their heads forward, or come to rest in awkward positions as their muscles are relaxed. Over time, this can cause physical discomfort, and subjects may be too relaxed to move or even be unaware of any discomfort until they wake up.
This is easy to handle with a direct suggestion. Once the subject is in trance, give them the suggestion to move their limbs to where ever is most comfortable for them, and they’ll find that keeping the head balanced lets them sink into trance even deeper.
Unintentional compliance is when an inexperienced hypnotist will add a trigger or post hypnotic suggestion, and then be astonished at how well the subject actually follows the suggestion. The most common risk is that the hypnotist will yell "sleep" at a standing subject, and the subject will instantly collapse onto the floor, aka "flopping." Subjects have been known to do complete somersaults from a sitting position as they go from sitting up in their chair to leaning forward, to overbalancing and landing on the floor, and still not wake up.
A subject told to be frightened of spiders may run out of the room, and the house. A subject told that they are walking in the woods and smelling flowers may have an allergy attack. It’s easy to underestimate hypnosis, but we are all easily manipulable meat bags.
Negotiation and always staying close to your partner are the best way to gain experience without causing problems.
One possible experience of hypnosis is hypnotic disassociation. It’s a feeling that the body seems to follow suggestions without the mind being involved, so the mind is a "passenger" or "along for the ride." This can be disconcerting or upsetting for new subjects. Hypnotists can also provide a sub-par experience, either with suggestions such as the Kilgrave Smile or by using porn or romance novels as inspiration for scripts.
The most important counter to this risk is to pay attention. If your partner looks upset or unhappy, stop immediately and check in. Negotiation, pretalk, check in and safeties are all ways to ensure that your partner is enjoying the experience.
Abreactions are usually caused by a suggestion that brings up feelings or memories from the past, sort of like a post-traumatic triggering. These can range from cringing, to shaking, to crying and potentially panic attacks. Subjects may also "shut down" and become unresponsive to checkins and suggestions. These are rare when proper negotiation takes place, but they do happen, and they are far more likely when an untrained hypnotist starts trying to be a hypnotherapist and starts working with childhood trauma.
Abreactions can be handled by remaining calm and giving disassociation suggestions, i.e. "The scene is fading, you focus on your breath. Feel your weight on the chair, know you’re safe." Repeat this phrase until they’re completely calm and there’s no trace of discomfort. Then wake them up, provide them all the aftercare they need, and discuss it with your partner. That way you can know a bit more about what happened and also provide them reassurance that they didn’t do anything wrong by abreacting.
Some intense feelings of shame and guilt are also common in newbies with hypnosis kinks, even outside of a specific scene. Society has many hang-ups about hypnosis, and teaches people to be ashamed. After the first few sessions, it’s not unusual to find some long repressed feelings popping up unexpectedly.
It’s okay. You are not alone.
It is common for hypnosis to result in transference of feelings towards your partner, on both the part of the hypnotist and the subject. In English, if you have a crush on someone but you are "just friends" and they hypnotize you and make you feel good, you may wake up with a bigger crush on them.
Because hypnosis feels good and involves trust and a suspension of critical faculties, feelings are common. When hypnotism relieves pain or anxieties as well, it’s natural to feel grateful and appreciative. This is more of a problem in therapy than in recreational hypnosis, but it is something that may be unexpected or awkward depending on the preexisting relationship.
Unresponsiveness is when a subject will ignore suggestions, but is clearly in hypnosis and not asleep.
Unresponsiveness may be caused by abreaction, but also the perverse opposite: hypnosis can feel so good that subjects may not want to wake up even after the session is over and you’ve done the wakeup count.
There are two ways to handle this. The nice way is to ground them to the environment and suggest that as you count up from one to ten, you’ll tap their knee, and as you tap they’ll start to say the numbers with you, and when you reach ten they’ll be all the way awake. The not so nice way is to use a hypnotic threat to do something horrible, such as "you’ll never be hypnotized again" or "I’ve gone professional, and I am now charging at my hypnotherapist rate of $300 a minute. Sleep as long as you want."
Spontaneous amnesia is when a subject will wake up following a hypnotic session, and have partial or no recollection of what happened during the session. This can be counter-intuitive to the hypnotist, who is totally focused and aware of the subject’s reactions and interactions. Even more interesting, the subject may remember exactly what happened when placed back in hypnosis.
This is called state dependent memory. Humans are very good at compartmentalizing and contextualizing information, and because hypnosis is a mental state that can be very different from conscious thought, it can impede recall.
There is a similar phenomenon where your partner may remember everything during the session, but will forget most of it an hour later. This is because hypnotic sessions can form a natural event boundary, smiliar to walking out of a room. Event boundaries are part of the event horizon model of cognitive awareness, which is a fascinating read if you’re a psychology nerd, but probably less so for curious newbie hypnotists who want to talk about the thing that happened. The inverse of this is the Zeigarnik effect, which postulates that people remember unfinished or interrupted tasks better than completed tasks.
Spontaneous amnesia can be countered by giving suggestions to remember everything that happened in the session, and by prompting specific events to your partner to ensure it sticks in long term memory.
Using substances that alter mental states has been shown to increase hypnotic suggestibility, in the case of alcohol up to 69% (nice) and in LSD up to 65%. However, there are some obvious and not-so-obvious risks to using hypnosis with an altered mental state.
One non-obvious risk is that using drugs with hypnosis when you are a newbie hypnotist makes it much harder to see what a hypnotized person looks like and how to interpret their reactions. Likewise, for your partner, it can be harder to determine what what hypnosis feels like by itself.
Another non-obvious risk is that suggestions given in an altered mental state may only be effective while in that state. This is another example of state dependent learning, most often seen with alcohol, where someone who is drunk may do things that they only remember doing the next time they drink.
Finally, the obvious. Being drunk, high, or hallucinating comes with some inherent risks, and they can magnify all the other risks that come with hypnosis. Someone who is drunk is much more likely to flop unexpectedly, abreactions on LSD are not fun, and someone who is stoned can be lethargic and unresponsive.
My recommendation is that you avoid substances altogether while going through this guide, and only experiment after you have a good understanding of your reactions and have a plan for managing risks.
Some physical reactions are possible, especially initially. Headaches or a feeling of pressure in the temples are not unknown after trance, especially with eye fixation inductions that cause eyestrain and tension. The feeling of pressure may come from tensing the scalp and eyebrows subconsciously. This does become less common with time, but has many remedies as this is also a common meditation issue.
Also common in hypnosis is hypnic jerk, a jerk or twitch of the muscles that usually occur when the person is going to sleep. This is completely harmless, but can be unexpected for newbies.
Lacrimation (eye watering) and injected sclera (eye redness) is common and harmless, although I don’t know the exact cause.
For most people, the heart rate slows down when in hypnosis. For people with a hypnosis kink, it’s not uncommon for heart rate to go up when being hypnotized, especially for the first time. This is normal enough to be a cliche, but is still worth pointing out since many hypnosis scripts that don’t take that into account.
Also worth mentioning is that people with a hypnosis kink, especially teenagers, should be aware that they may become aroused during hypnosis, and it may be very visible to their partner.
One of the risks of a hypnotic induction is that you can do everything right and it may still not work. This guide will work for most people, but it does make assumptions about your partner.
Many hypnotic inductions assume a certain mindset and background that may not be universally applicable. This guide will work for most people, but not all, and it’s usually due to the way that people process information.
Some people are visual thinkers, and have difficulty with suggestions involving abstract concepts. Some people have aphantasia and have no visual component to thinking at all. Some people are non-verbal, and respond better to kinesthetic inductions and physical deepeners than someone talking at them. A person with dyslexia may have problems losing letters from the alphabet. A person with attention deficit disorders may become distracted or bored with a progressive muscle relaxation. A person with anxiety or control issues may be uncomfortable with how suggestions manifests for them. And a person with high-functioning autism may find confusion inductions incredibly annoying.
The key thing here is that there is no one magic incantation of an induction that will work for everyone. People who process information differently may need a different approach to hypnosis that is effective for them. Good hypnotists tailor to their subjects, rather than trying to make their subjects fit themselves into to a process.
For your partner, take a look at the Practice section at the bottom of this, which has tips written by "analytical subjects" (a term I don’t personally like) who have found things that work for them.
Graham Old has a book Hypnosis for the Hard to Hypnotise that goes over situations and techniques more specifically.
Intemperance is one of the lesser known risks of hypnosis. It comes into play not when things go wrong, but when everything seemingly goes right.
There can be many wants and desires tied up with fantasies of hypnosis, and realizing these desires and "having control" can be intoxicating, especially if you have very little power over your outside circumstances. Likewise, your partner may have anxieties and responsibilities that hypnosis can lift off their shoulders. There is a temptation to avoid uncomfortable feelings or situations by doing hypnosis more, and using hypnosis as an escape. You may try to master hypnosis completely, or your partner may attempt to remain in hypnosis forever. This is unhealthy, and it rarely ends well.
Feelings are important, and should be brought out into the open and talked about, even (especially!) if it’s uncomfortable to do so, and even if it means slowing down or pausing for a time. Do one thing at a time until you’re sure you can handle that thing.
The final risk is manipulation and abuse: deliberately doing something that wasn’t negotiated or pushing past negotiated limits.
It’s often said that hypnosis can’t make you do anything you don’t want to do. This is true for the vast majority of people, but people can make you do things you don’t want to do. If you’re in a room with someone who is insistent, persuasive and who is larger than you, then even if you’re fully conscious you may feel pressured, trapped, and unable to refuse. There is also a psychological tendency to freeze and disassociate under threat, which can be exacerbated by hypnosis.
There are stories of hypnotists "joking" with tropey "you have no will, you must obey" language or role playing a scenario. They may say that they didn’t realize how real and terrifying it is to the subject, but this isn’t an excuse. They ignored consent because they thought it was funny.
There’s also an implied caveat. While most people can’t be made to do things they don’t want to do in a single session, there is also a very small minority of people who are highly hypnotizable and who may find it difficult to resist following even unwanted suggestions. Unfortunately, there’s no litmus test that determines how suggestible you are before you are actually hypnotized.
Insist on safeties and be clear that you’re getting up and leaving if it doesn’t work for you. You are totally within your rights to ensure the session is recorded or have a friend sit with you during the session, especially if you don’t know your partner all that well. Trust has to be earned.
There’s also a big difference between hypnosis between strangers and hypnosis with someone who you trust, or who has authority over you in some way. There are risks to letting someone into your head. If you are in a relationship with an emotional abuser, all the tools of influence can be used in hypnosis to confuse, reframe, or gaslight you. With enough rapport and trust and repeated sessions, they can change what you want to do. There have been documented instances of trusted authority figures using hypnosis to aid in sexual assault.
There’s unfortunately only so much this guide can do to describe predatory behavior, as a skilled manipulator will build up trust by being completely trustworthy until you let your guard down. There is an excellent book, Why Does He Do That, that discusses manipulative behavior and "tells" that abusers have.