The health effects of cannabis and cannabinoids (part I)

By Raquel Peyraube

Dr. Raquel Peyraube is a doctor in medicine and a specialist in the problematic use of drugs. She has trained in psychiatry, toxicology and psychoanalytical psychotherapy and in subjects such as childhood, adolescence and social exclusion. She has 28 years' experience in the field. Throughout her career, she has made contributions in training, prevention, treatment and damage reduction, including innovating theoretical and methodological developments with emphasis on ethical issues. As a former clinical director of ICEERS, she is now an ad hoc consultant of the Uruguayan National Drugs Secretariat giving advice on reform of the public drugs policy and of the Institute of Cannabis Regulation and Control. She is a clinical researcher and a member of research teams for monitoring the law in Uruguay. She sits on several international scientific committees and is an active member of the IACM (International Association for Cannabinoid Medicines). She currently works on the development of clinical trials, medical education on medicinal cannabis, and dissemination of information and advice for reform of drugs policies in various countries.

When discussing the effects of cannabis on health, there is a tendency to refer only to its adverse effects and to describe it exclusively as a “substance of abuse”. Like any substance –including medicine– cannabis can have adverse effects. However, although there may be issues associated with usage and chemical dependency –as there are with certain types of medication and other legal and illegal substances– cannabis and cannabinoids also have beneficial effects.

In the first part of this article, we are going to classify the effects of cannabis under different headings. Throughout the different sections, we'll examine these effects in the same way as medical literature treats any substance: reviewing the beneficial and adverse effects, taking into account the fact that an effect that might be considered adverse in one usage situation could be exactly what is intended in another (for example, enhanced appetite or alteration of perception and memory).

Another important aspect to consider when discussing the effects is the harm-to-benefit ratio; in other words a comparison of the relative extent of any benefit and any adverse effects. As an example, how important is a dry mouth when cannabinoids are being used to treat life-threatening refractory epilepsy? Or temporary drowsiness in treatment to accompany chemotherapy for cancer?

We should note that the effects of the cannabis plant differ from those of cannabinoids (the plant’s most important active principles) taken in isolation. The plant also contains other compounds, terpenes, which as well as giving it its distinctive smell, have effects that complement or modulate those of the cannabinoids.

Finally, although the pursuit of pleasure is an important component of mental health, we shall not deal here with the effects sought by non-medical use.

The effects of cannabis can be classified using a number of different variables:


  • Beneficial – Harmful
  • Intended – Unintended

Strictly speaking, cannabis is not a “substance” but a plant. It would be more accurate to speak of the effects of the “substances”, since many different compounds are taken into the human body when cannabis is consumed, several of which have an impact on the effect.
Whether an effect is classed as beneficial or harmful and intended or unintended will depend on the circumstances and the purpose of its use. For example, in the case of PSTD (Post-Traumatic Stress Disorder), memory disruption is actually an intended effect, since the purpose of treatment is to help patients forget and/or detach themselves emotionally from the traumatic situation that caused the disorder. Thus, in this case, what might be considered a harmful effect can actually be beneficial.


  • Respiratory (inhalation)
    1. 1. Inhalation of the smoke produced through the combustion of flowers and leaves (smoking)
    2. 2. Inhalation of vapours (using special devices)
  • Orally or by digestion (ingestion with swallowing of preparations)
  • Percutaneously (application of patches and creams on the skin)
  • Sublingually (sprays and tinctures applied under the tongue)
  • Rectally (use of suppositories)

In the future, as the technology advances, other forms of administration will undoubtedly be developed, such as eye drops and injectable solutions.

The choice of one form of administration or another depends on a number of different factors. Some routes are selected to achieve a systemic (general) effect, for example in the treatment of epilepsy, neurodegenerative diseases, nausea and vomiting, PTSD, immune disorders and chronic pain. These are the inhalatory, oral, sublingual and rectal routes.

Topical administration is the method of choice in the treatment of skin disorders such as acne, and others conditions which are manifested on the skin, such as psoriasis. It may also be used in the treatment of localised muscle or joint pain.

Finally, for some clinical conditions – such as osteoarthritis and rheumatoid polyarthritis– it may be deemed necessary or more beneficial to use both systemic and topical routes complementarily.

The speed with which the treatment begins to take effect and the duration of the effects varies from one route of systemic administration to another. This factor must be taken into account when deciding on a therapeutic strategy. In the case of the inhalatory route, the effect is very swift and short-lasting, meaning that the dose has to be repeated frequently. In contrast, the digestive route can take up to two hours to work, but the effect lasts many hours, so treatment need only be administered twice or three times a day.

Each route of administration has its advantages and disadvantages, its usefulness and its application. However, smoking is clearly not recommended for the same reasons as smoking tobacco is advised against: it produces carcinogenic compounds and other disorders.


  • Action on a psychopathologically healthy structure
  • Action on a psychopathologically vulnerable or unhealthy structure

This is another equally important aspect. Cannabis and cannabinoids, particularly THC, act differently in these two psychopathological structures. For example the much-feared “schizophrenia-inducing” effect attributed to the substance (which we shall discuss in Part II), does not present in all cases, but only in patients with an existing vulnerability, where it can advance onset of the disease.

Despite the risks of consuming THC for people with a certain vulnerability, some cannabinoids (such as CBD) have a therapeutic effect in certain mental health disorders, such as anxiety, depression, panic attacks and psychosis – in other words, among patients with an underlying mental disorder.

Therefore, what is important is not only the base structure, but also the cannabinoid and association of cannabinoids used. Varieties with a high psychoactive potency are more inclined to have adverse mental effects. The effect also depends on the dose; it is not uncommon to encounter cases of people who have previously consumed cannabis recreationally on many occasions with no adverse effects, but who experience temporary psychotic episodes when they take an excess dose.

Based on my clinical experience, I would like to stress that the use of cannabis for medicinal purposes by a person with a psychopathologically vulnerable structure, although requiring great caution, need not inevitably be accompanied by adverse effects.

Finally, we also need to consider the possibility of idiosyncratic reactions: i.e. reactions which are not foreseeable or characteristic of the product, but which may result in unforeseen effects in certain individuals. For all of these reasons, it is advisable for anyone beginning to use cannabis, for whatever reason, to start with a low dose, increasing it gradually as necessary until they find the best dose for their purposes with the least adverse effects.


  • Medicinal usage (in the cases of disease with therapeutic purposes)
  • Non-medicinal usage (adult and recreational)

It is important to note that the effects can also vary depending on the type of use. Due to a variety of factors –including the product itself, the means of administration and dose, the pathology to be treated, susceptibility to the effects of cannabis– the effects that are recognised in non-medical use are not generally experienced when cannabis is used medicinally under the supervision of a qualified and experienced doctor. This is the case of dependency, which is very infrequent in my clinical experience. Likewise, the psychoactive effects can be avoided even when THC is used.

EFFECTS RELATED TO THE “SET AND SETTING” (i.e. (the mindset of the individual and the context)

“Set and setting” factors are related to the issues discussed above in Groups 3 and 4, but particularly to aspects of the person and the context of use. The effects of cannabis are not the same when it is taken in a secure context, in pleasant surroundings, accompanied by people with a good understanding, as when it is taken in a situation of prohibition and censure with a police presence. The probability of adverse effects and feelings of persecution are greater in the latter case.

As for the “set”, individuals who have negative expectations or fears should not consume cannabis since the probability of adverse effects in the psychological sphere are increased.


  • Natural cannabinoids in plant extracts
  • Purified natural cannabinoids in artificial preparations
  • Synthetic cannabinoids in pharmaceutical formulations
  • Synthetic cannabinoids in recreational use

In both medicinal and non-medicinal use, there are different effects, levels of tolerance and risks for these four types of product. Synthetic products have the greatest risk and adverse effects, some of which can be severe and even life-threatening due to their cardiovascular impact. This is true of synthetic cannabinoids used for recreational purposes from the group of substances known as “legal highs”. Synthetic cannabinoids that have been accepted for medical use (Dronabinol, Nabilone) also have adverse effects, but do not pose the same risk as those sold on the clandestine market.

As can be seen, any discussion of the effects of cannabis and cannabonoids is neither as simple nor as absolute as is often suggested.

In the second part of this article, we will examine some of the adverse effects that cause the greatest concern and are frequently advanced as arguments against the legalisation of cannabis.

Part 2 »

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