What is THC, and how does it work in the body?

So, how does THC work in the body? 

Its chemical structure is remarkably similar to substances that our own bodies naturally produce. This allows THC to interact with the endocannabinoid system, influencing mood, pain, appetite, sleep, and memory. This overlap between plant chemistry and human biology sits at the heart of THC science.

The more research that is conducted (and, thankfully, with cannabis legalisation sweeping the globe, research is ramping up at almighty pace), the clearer it becomes that THC is not simply about intoxication. The emerging, and extremely fascinating research is proving that THC can have a marked positive therapeutic effect for chronic pain, sleeping issues, women’s health issues, mental health concerns and neurological disorders.

Try Releaf’s medical cannabis eligibility checker. It’s free, takes less than 20 seconds, and gives you a clearer idea of your potential eligibility.

What is THC?

In simple terms, THC is a plant-derived molecule with a structure similar to the cannabinoids our bodies make. Cannabis strains can produce THC in high concentrations, while hemp cultivars do not (although hemp does offer up high levels of CBD). This difference explains why hemp does not have the intoxicating effects associated with cannabis.

When people talk about THC in cannabis, they are usually referring to the main psychoactive compound that causes the “high.” It is also the form most often studied for medical applications. Some articles mention THC-A, the acidic precursor that converts to THC when heated, but THC itself remains the best known and most researched version. 

Cannabis plants also produce other cannabinoids such as CBD, CBG, CBC, and CBN, but none show the same combination of psychoactive and therapeutic effects as THC.

THC and the endocannabinoid system

The endocannabinoid system (ECS) was discovered in the late 1980s and is now recognised as one of the body’s main regulatory networks. It helps maintain balance (or, to be more precise, homeostasis) and influences our perception of pain, how our metabolism works, our immune responses, sexual health, and memory.

The ECS has three parts:

• CB1 and CB2 receptors
• Endocannabinoids, which are messenger chemicals made by the body
• Enzymes, which break them down once their role is complete

The body does not produce THC. What it does produce are its own cannabinoids, like anandamide. THC is structurally similar enough to interact with the same receptors and influence many of the same processes.

CB1 vs CB2 receptors

Thanks to the structural similarity between THC and anandamide, THC can bind directly with the CB1 receptors, which are mostly located in the central nervous system (the brain and spinal cord). This explains why cannabis can shift mood, sharpen or dull appetite, bend perception, and trigger memory changes. At the same time, CB1 activity is what makes THC useful in medicine, supporting pain relief, better sleep, and mood regulation.

CB2 receptors are found in the peripheral nervous system, with the highest numbers in the immune system and peripheral tissues. When activated, they do not produce intoxication, but they shape how the body responds to inflammation and immune stress.

How does THC produce effects in the body?

Once THC binds to receptors in the endocannabinoid system, it changes how those receptors send signals through the body. This is why one compound can influence mood, perception, appetite, inflammation, and pain all at once. 

The outcome depends not only on the dose but also on how THC is taken, which affects both onset and intensity.

Psychoactive and physiological effects of THC

THC can shift how we feel, and how we respond to external stimuli, in a range of ways. 

Many people report euphoria, deep relaxation, and changes in how time and sensory input are experienced. Appetite stimulation is common, often called “the munchies,” but it can also ease nausea and settle the stomach. At the same time, THC interacts with pain pathways, which is why it has been studied for analgesic effects in both acute and chronic pain. Some patients also describe muscle relaxation, lighter mood, or improved ability to fall asleep.

These immediate effects explain both the recreational reputation of cannabis and the growing medical interest in THC for conditions linked to pain, sleep, appetite, and stress.

Differences in THC dose and administration response

The way THC feels is shaped by both the amount taken and the method of administration, but there is no single response that applies to everyone. Some people find that small, carefully measured amounts help with pain, sleep, or anxiety. Others may tolerate higher amounts well and even find stronger doses more effective for certain symptoms.

This variability is why medical cannabis is introduced gradually through a titration process, always under the guidance of a specialist doctor.

Researchers often describe THC as having “biphasic” effects, meaning it can produce different outcomes at different levels, but the reality is more individual. What might feel calming for one person at a given dose could feel overstimulating for another.

Clinical oversight and patient feedback are what make it possible to find the balance point where benefits are maximised and unwanted effects minimised.

Administration also plays a role: 

• When inhaled (through a medical grade vaporiser - smoking medical cannabis is illegal in the UK), THC reaches the brain in less than a minute, and the effects are felt almost immediately. 
• Sublingual administration (where medical cannabis oil placed is under the tongue a minute or two) usually takes 10 to 15 minutes for the effects to become fully noticeable. 
• Swallowed forms take the longest to take effect (60 to 90 minutes) because THC must pass through the digestive system and liver before entering the bloodstream. This “first-pass” metabolism slows absorption but can also extend and intensify the effects once they appear.

You can read more about how THC levels are prescribed in our guide: Understanding THC levels in medical cannabis in the UK.

The science behind the potential therapeutic benefits of THC

Before we dip our toes into the potential therapeutic benefits of THC, it is important to point out that even though there are many promising research papers and clinical trials already published, this remains an emerging field. Thankfully, with medical and recreational cannabis legality slowly opening up at a global level, researchers are finally researchers now have greater freedom to study THC in depth. This is leading to larger trials, better data, and a clearer picture of how different THC percentages may support patients.

It is also important to point out that THC, and medical cannabis as a whole, should never be regarded as a first-line replacement for conventional medications, but rather as an adjunct treatment option that can be considered if and when first-line treatments fail.

That said, THC looks to be a great complementary option for a surprisingly wide range of medical conditions, with research indicating that it is a potential treatment for pain, inflammation, neurological disorders, glaucoma, nausea, mental health issues, and even muscle spasticity.

THC for pain

Pain is usually split into three main categories: acute, chronic, and neuropathic.

• Acute pain is the most common type of pain, including injuries like sprains or strains, or post-operative issues. It typically lasts for days or weeks before gradually subsiding, during which the body heals naturally.
• Chronic pain is a persistent type of pain that lasts for months or even years and is often linked to conditions like arthritis, fibromyalgia, back issues, and migraines. Managing everyday life becomes challenging due to ongoing discomfort.
• Neuropathic pain, also known as nerve pain, occurs when the nervous system is damaged or irritated due to various factors such as injuries, infections, or medical conditions. This type of pain is characterised by shooting sensations, numbness, tingling, and a burning or electric shock-like feeling. It is crucial to understand the underlying causes and seek appropriate medical treatment in order to effectively manage and find relief from neuropathic pain.

When it comes to acute pain, THC has shown the potential to offer rapid relief without the need for any additional medications.

One clinical medical review, which looked at a range of differing back pain-related issues from a total of 110 patients, found that medical cannabis application proved to be extremely efficient, while heavily reducing the need for opioid medications. This is an especially important discovery given the current opioid epidemic in the UK and the US.

Studies on delta-9 THC for pain show it may reduce the intensity of chronic and neuropathic pain while lowering the need for opioids. For chronic pain, there are a range of studies we can look at, but this systematic review does so for us. Focusing on twenty-five separate studies, it found that:

“Cannabis products with high THC-to-CBD ratios and sublingual, extracted cannabis products with comparable THC-to-CBD ratios may be associated with short-term improvements in chronic pain”

And for nerve pain, the results also look promising. In a randomized controlled trial from 2010, titled "Smoked cannabis for chronic neuropathic pain", it was discovered that a single inhalation of 25 mg of 9.4% tetrahydrocannabinol herbal cannabis, three times daily for five days, effectively alleviated pain intensity, enhanced sleep quality, and exhibited safe and high tolerability.

THC for glaucoma

Glaucoma is a complex disease that is not typically caused by a single issue, but rather a combination of conditions that can lead to vision impairment or even complete blindness. It ranks among the top causes of blindness in adults and can be classified into three main groups:

1. Primary Glaucoma: There are two primary types of glaucoma - primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG). These are the most common forms.
2. Secondary Glaucoma: This type of glaucoma occurs as a result of other medical conditions or trauma.
3. Developmental Glaucoma: Glaucoma that affects children and infants. It is an extremely rare form, with only around 5 in 100,000 children being affected.

THC application may help reduce the severity of glaucoma through two separate mechanisms.

Firstly, THC can reduce intraocular pressure (IOP) in the eye due to its ability to dilate blood vessels and decrease fluid production, reducing inflammation in the eyes which can help slow down the progression of glaucoma.

Secondly, the cannabinoids that interact specifically with the CB1 receptor, such as THC, have shown promising therapeutic potential for addressing corneal surface damage and alleviating associated pain. By targeting the CB1 receptor, these cannabinoids may help modulate the signalling pathways involved in corneal healing and provide relief from discomfort, offering a potential avenue for the development of novel treatments in this field.

THC for the reduction of nausea and vomiting

One of the earliest medical applications of cannabis was in alleviating nausea and vomiting, particularly in cancer patients undergoing chemotherapy. 

In 1986, the US Food and Drug Administration (FDA) approved a synthetic form of THC for patients undergoing chemotherapy, to help address the heavy nausea that often goes hand in hand with the treatment. 

And the science has only grown since then. 

In the UK, the NHS can prescribe synthetic cannabinoids such as Nabilone for chemotherapy-induced nausea and vomiting in cases where other medicines have not worked. Unfortunately, in practice, the NHS has been historically mostly unwilling to offer many prescriptions, even with the shift in legality

One of the more recent clinical trials, titled "Regulation of nausea and vomiting by cannabinoids" found that activating the CB1 receptor had a marked affect on the reduction of nausea and vomiting when compared to a placebo group. The results showed that THC had a clear antiemetic effect, helping reduce the severity of symptoms in both healthy individuals and cancer patients undergoing chemotherapy.

THC for cancer-related support

Cancer treatment saves lives, but it can also take a heavy toll. 

Pain, sickness, broken sleep and the anxiety that builds with each appointment are all too common. Conventional medicines such as chemotherapy, opioids, steroids and anti-nausea drugs are essential, but they can also create new problems of their own. For some patients the relief is incomplete, leaving quality of life pushed to the background.

This is where medical cannabis for supportive cancer care is making inroads. 

In the UK, synthetic THC medicines such as nabilone are licensed for chemotherapy induced nausea and vomiting when standard anti-nausea drugs are not enough. A 2021 expert review, “The Management of Cancer Symptoms and Treatment-Induced Side Effects With Cannabis or Cannabinoids,” highlighted the wider potential of cannabinoids in cancer care.

In animal models, THC was shown to “reduce multiple kinds of pain and amplify the effect of opioids, meaning lower doses were needed.” Trials in humans have been mixed, but some patients using nabiximols (a 1:1 blend of THC and CBD) reported “modest improvements in pain scores and sleep.” 

The same review also noted that many patients described cannabis as improving anxiety, sleep and overall outlook during treatment. Participants in the study summed it up as a “positive shift,” describing less fear and more calm. These changes may seem small on paper, but for patients living through intensive cancer care, they can make daily life far more bearable.

The evidence is still early, and results are not consistent across all trials. What stands out is that outcomes depend on dose and delivery. Lower, titrated amounts of THC are more likely to ease anxiety and sleep problems, while larger amounts are not always better. 

THC for neurological conditions

Neurological disorders such as multiple sclerosis (MS), Parkinson’s disease, Alzheimer’s disease and epilepsy are some of the most complex to manage. Conventional treatments can definitely ease certain symptoms, but not all patients find the symptomatic relief they so sorely need. Research into THC has opened the possibility of a new supportive pathway.

One large scale study that focused on MS patients found that throughout the 15-week trial, participants that were given oral THC extract showed

"There was evidence of improvement in walking time for ambulatory patients and in patient perceptions of spasticity, muscle spasms, pain, and sleep" and that "There was evidence of a treatment effect on patient-reported spasticity and pain (p=0.003), with improvement in spasticity reported in 61%"

In Parkinson’s disease, smaller studies have suggested THC may help with tremors, stiffness and pain, although results remain mixed. In Alzheimer’s disease, laboratory work has indicated cannabinoids could protect neurons from damage and reduce brain inflammation, with some early evidence for improvements in agitation and sleep.

Epilepsy research has focused mainly on CBD, but THC has also been investigated in smaller trials, with some patients reporting fewer seizures when it was used alongside other treatments.

Perhaps the most intriguing line of research is THC’s potential role in neurogenesis, the creation of new brain cells. Once thought impossible in adulthood, this process is now recognised as lifelong, and preclinical studies suggest THC could enhance neurogenesis and support memory and learning.

At present, the evidence for THC in neurological disorders is promising but incomplete. Its role is best seen as complementary, offering potential relief for difficult symptoms under careful clinical guidance while larger trials continue.

THC for anxiety and mental health

THC’s relationship with mental health is complex, but its potential application is tentatively supported by the emerging evidence. 

At lower prescribed doses, it has been shown to ease anxiety, improve mood, and reduce the stress response. Some patients also report benefits for depression and post-traumatic stress disorder. These effects are thought to come from THC’s ability to mimic anandamide at CB1 receptors in brain regions tied to mood and emotional regulation.

A 2020 clinical review, “Cannabinoids and their therapeutic applications in mental disorders”, brought together findings from dozens of studies on cannabinoids for conditions including depression, anxiety, and PTSD. 

The review noted that while more controlled trials are needed, cannabinoids such as THC show promise as complementary treatment options. In particular, low potency THC preparations, often balanced with CBD, were linked to improvements in anxiety and trauma related symptoms in some patients.

It is important to note, however, that THC is not suitable for everyone. In people with a history of schizophrenia, psychosis, or a strong family tie to these conditions, THC can increase the risk of relapse or worsen symptoms. 

Because of these risks, UK prescribing guidelines exclude patients with schizophrenia or psychotic disorders to ensure treatment remains safe and appropriate.

How is THC used in the UK medical context?

Medical cannabis was legalised in the UK in 2018. NHS access is still very restricted, so most patients prescribed THC-based medicines do so through private clinics. 

Unlike recreational cannabis, medical cannabis products are strictly regulated, lab tested, prescribed by specialists listed on the GMC Specialist Register, and dispensed by licensed pharmacies. 

They are prescribed to relieve specific symptoms, not to cause intoxication, with ongoing clinical guidance and oversight. 

Releaf prescribing safeguards

At Releaf, THC prescriptions are guided by clear safety rules. Patients under 25 are not prescribed products containing more than 25% THC. Extra caution is taken where there is a history of psychiatric illness. 

Schizophrenia and psychosis are exclusions, while bipolar disorder requires careful assessment and usually support from the patient’s GP or mental health team before treatment can be considered.

For more details on how high THC levels are defined, see our guide: What percentage is considered high THC?

The Releaf patient journey

The Releaf pathway begins with our free medical cannabis eligibility checker, which takes less than 20 seconds and helps patients understand if they may qualify for treatment. 

If you are found to be potentially eligible, you will be prompted to create a free Releaf account, upload your medical records (which is easily done thanks to our integration with the NHS Spine), and then book in for an initial consultation with a specialist doctor. 

During this consultation, the specialist will carefully consider your full medical history, previous treatments, and current symptoms. If medical cannabis is deemed appropriate, a personalised prescription will be created for you.

Once prescribed, you will gain access to Releaf’s bespoke digital health platform. This allows you to order and track prescriptions, arrange repeat supplies and follow-up appointments, and manage your treatment plan in one place. All medication is dispensed through licensed UK pharmacies and delivered directly to your door.

Patients are also issued a Releaf medical cannabis card, which provides official proof of prescription. This card can be shown to employers, travel authorities, or law enforcement if needed, offering reassurance and protection in daily life.

FAQs

What does THC do in the body?

THC interacts with the endocannabinoid system, a network of receptors that regulate mood, pain, sleep, appetite, and memory. By binding mainly to CB1 and CB2 receptors, it can ease chronic and neuropathic pain, improve sleep, stimulate appetite, and support mental health by reducing anxiety or stress in some patients. Early research also points to potential benefits in neurological conditions such as multiple sclerosis, Alzheimer’s, and epilepsy.

Is THC safe for medical use?

When prescribed by a specialist doctor and taken in controlled doses, THC is generally considered safe. Clinical oversight is essential because responses vary, but with gradual titration patients often find effective relief for pain, sleep, and mental health without significant unwanted effects.

Can THC reduce chronic pain?

Yes, research shows THC can help with chronic and neuropathic pain, offering relief for conditions like arthritis, migraines, or back pain. Clinical studies also suggest it may reduce reliance on opioid medicines, making it a valuable complementary option for long-term pain management.

What are the short-term effects of THC?

Short-term effects can include relaxation, euphoria, altered perception, and increased appetite. Many patients also experience improved sleep and reduced pain intensity.

Does THC cause any side effects?

Yes, THC can cause side effects, though for most patients they are mild and manageable. Dry mouth, dizziness, or a touch of drowsiness are the most common. Some people notice their short-term memory feels a little off, while others may feel anxious if the dose is pushed too far.

THC can also briefly increase heart rate and blood pressure. Patients with cardiovascular conditions should discuss this with their specialist before starting treatment.

These effects are usually temporary and, with a gradual titration plan overseen by a specialist, most patients find a level where the benefits outweigh the drawbacks.

Accessing medical cannabis treatment with Releaf

Releaf is the UK’s fastest-growing, most reviewed, highest rated, and most trusted medical cannabis clinic. With a world-class clinical team of more than 30 doctors and specialists, supported by over 30 dedicated Patient Support Team members (fully UK based), we provide the expertise and care needed to guide patients safely through treatment.

Every prescription is thought out with care and personalised to each individual patient, dispensed through licensed UK pharmacies, and supported by Releaf’s built-from-the-ground-up health tech platform and medical cannabis card, giving patients confidence and protection in daily life.

You can start today with our medical cannabis eligibility checker. It is free, takes less than 20 seconds, and gives you a clear answer on whether medical cannabis could be an option for you.