Side-effects of cannabis use

Health > Information for patients

In general, patients seem to tolerate medicinal cannabis well: the possible side-effects of cannabis and cannabinoids are of mild severity, and usually disappear within several hours. When properly used cannabis does not have any remaining physical toxic effects. Nevertheless, it is important to know what may happen when you ingest cannabis as a medicine. Unwanted side-effects mainly occur after consumption of high doses, or when cannabis is used in combination with other substances that potentiate its effects (e.g. alcohol or some medications). As lower doses of cannabis are often sufficient to induce a medicinal effect, serious side-effects are rarely encountered in the Dutch medicinal cannabis program.

Know your quality

The use of medicinal cannabis can generally be considered as safe, provided that the quality of the product is controlled. When cannabis of unknown origin is used, there is always a chance the product is contaminated with pesticides, growth enhancers, heavy metals or microbes.

Preventing cannabis side-effects

Unwanted and unsuspected side-effects may be prevented by following some simple advice: start the treatment with a low dose, and take plenty of time to increase till the optimal dose is found. This process can take up to 2 weeks, increasing the dose a little each day. To what extent side-effects may occur also depends on the mode of administration chosen (oral vs. inhalation). The first few times of use of medicinal cannabis, it is recommended to be in a calm and safe place (e.g. your own house) without disturbances. Be aware it may take a few hours before the effects of cannabis wear off. It is good for the patient to have a trusted person around to support him/her during the experience.

Some of the potential negative consequences of (medicinal) cannabis use are listed below. Patients with a hereditary risk of psychosis or other psychiatric problems (e.g. problems with schizophrenia or depression in the family) should first discuss this with their physician, before trying cannabis as a medicine. The same warning applies to patients with cardiac/coronary problems.

Most common weed side-effects

The common acute effects of cannabis, occurring quickly after consumption are: dry mouth, reddened eyes, dizziness, extraordinary appetite, euphoria. Responsiveness/alertness of the user may be reduced, especially in the few hours directly after consumption. Furthermore, users may experience increased heart-rate and a lowering of blood pressure. This can cause a sudden dizziness and weakness, so it is advised to be aware of falling. Consequently, people with cardiac or coronary problems should always consult their physician before trying cannabis as a medicine. In general, all effects mentioned will spontaneously dissolve within a few hours.

Getting ‘high’

When using larger doses of cannabis (marijuana), a user may experience becoming ‘high’; a situation of intoxication  where the experience of reality becomes distorted. The main component of cannabis, the cannabinoid THC, is responsible for this. The chance of becoming high is greater when cannabis is consumed orally, because the digestive system releases chemicals (THC-metabolites) which are even more potent than THC itself. This is one of the reasons why edibles commonly lead to problems with overdosing. When consuming cannabis in the form of tea (liquid, which is easier to digest) such problems occur less frequently. Patients susceptible for the psychotic side-effect are advised to use cannabis variety Bediol, as CBD is capable of suppressing this effect of THC.

Most often, the feeling of being ‘high’ is experienced as euphoria, where the person feels extraordinarily happy and energized. As time passes this situation may change into feeling deeply content and relaxed. However, in some individuals, and depending on the cannabis product consumed, this altered experiencing of reality (intoxication) may lead to confusion and panic. Most often it is sufficient to sit or lay down in a calm and familiar location, preferably with someone familiar to talk to. In case there is an experience of an unwanted ‘high’, remember it can be simply prevented next time by consuming less cannabis, or by taking more time to administer the dose.


Every now and then, there are new reports on the effects of cannabis on psychosis. How these two are linked, however, is scientifically not completely clear yet. The discussion basically revolves around a chicken-and-egg problem: does cannabis induce psychosis in otherwise totally healthy individuals, or does someone need a pre-existing genetic vulnerability for psychosis before cannabis use brings it out into the open?

Recent scientific studies into this matter suggest that a small proportion of the population carries a specific gene. Carriers of this gene have an increased risk of developing psychotic problems when using cannabis (as a medicine or otherwise). Patients interested in using medicinal cannabis for the first time, and who have a (family) history of psychotic problems, should always discuss this with their physician.

Health risks related to smoking marijuana

Smoking cannabis (with or without the presence of tobacco) as a way to use medicinal cannabis is not advised. During the combustion of cannabis, a range of harmful and toxic compounds is released, including ammonia, carbon monoxide and tar. These chemicals are inhaled and therefore end up in the lungs or bloodstream. Especially for chronically ill patients, this may not be the preferred way of administering a medicine, and the harm may be greater than the short-term benefits. Fortunately, medicinal cannabis can be consumed in a variety of other administration forms.

Dependence and tolerance

Although lots of studies have been performed on the recreational (drug) use of cannabis, not much is known yet about dependence (addiction) in the case of medicinal use of cannabis. In many cases, small amounts of cannabis are already enough to experience the desired medicinal effect. In such a situation use of cannabis does not easily lead to a situation of addiction. Dependence may however evolve when large amounts of strong cannabis are used, for example when several grams of the variety Bedrocan are needed daily to treat your condition. In such a situation, reducing or stopping your daily intake may lead to withdrawal symptoms such as mild forms of restlessness, irritability, insomnia, and nausea.

Tolerance occurs when the body gets used to a medicine so that more medicine is needed to get the same response as before. There are no real scientific clues that repeated use of cannabis leads to tolerance in large groups of patients. It is therefore unlikely that you will need steadily increasing amounts of cannabis to obtain the desired medicinal effect.


Chronic cannabis use during pregnancy may have an effect on the unborn child. The active components in cannabis, and more specifically THC, may be transferred to the fetus, and will appear in the mother’s milk. There are scientific indications that exposure to cannabis in the womb may alter the development of children. The use of medicinal cannabis during pregnancy and lactation is therefore strongly discouraged. For more information please contact your physician, or the Office of Medicinal Cannabis.


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