The combination of the ongoing opioid crisis, inappropriateness of long‐term use of
NSAIDs, limited efficacy and tolerability of existing analgesics and the fact that
no new analgesics (except for anti‐migraine medications) were launched during the
past two decades, has considerably depleted the arsenal of chronic pain pharmacotherapy.
No wonder therefore, that the old‐new player, medicinal cannabis (MC), has erupted
onto the pain field. Indeed, MC, which consists of herbal cannabis (HC), in either
inflorescence or oil extract forms, and cannabis‐based medicinal products (CBMP),
is being increasingly used in chronic pain management. As a result of public pressure
by media, patient advocacies and political lobbyists, MC has bypassed established
routes of regulatory approval in many countries. In addition, many unregulated cannabis‐based
products lacking robust efficacy and safety data, especially cannabidiol (CBD) containing
preparations, are readily available in some countries.
Basic research has promoted considerable understanding of underlying mechanisms and
sites of action of MC in nociceptive systems. Similarly, substantial evidence from
preclinical studies in animal models of pain (nociception) support the notion than
MC holds promise as effective analgesics in chronic pain (Häuser et al., 2018). However,
translation of these observations into solid clinical evidence for the efficacy of
MC in chronic pain—based on high quality randomized controlled trials (RCTs)—remains
elusive. Altogether, around 60 RCTs were published so far, varying considerably in
population sizes and characteristics (e.g. chronic vs. neuropathic or cancer pain),
the administered MC (ranging from herbal cannabis to synthetic Δ9‐trans‐tetrahydrocannabinol
(THC)), dosages and ratios of the main components THC and CBD, route of administration,
duration of treatment (hours to months) and primary outcome measures, thus yielding
equivocal results. Attempting to consolidate the results, systematic reviews and meta‐analyses
(SRMAs) began to emerge. Between 2010 and summer 2019, 57 such articles were published
but confusingly, provided a wide scale of conclusions ranging from clear evidence
for efficacy to the exact opposite. In a recent high‐quality SMRA, only 36 of the
RCTs met inclusion criteria, due to significant methodological faults, and those too
had high and/or uncertain risk of bias (Fisher et al., 2021). As a result, the International
Association for the Study of Pain (IASP) released a position statement in March 2021
declaring that due to a lack of evidence from high quality research, it does not endorse
the general use of cannabinoids to treat pain. Yet, IASP also stated it does not wish
to dismiss the lived experiences of people with pain who have found benefit from their
use (IASP, 2021). In contrast, based on only slightly different exclusion and inclusion
criteria, another recent high‐quality SRMA of 32 trials concluded that there was moderate
to high certainty evidence that non‐inhaled MC produced a small to very small improvement
in pain relief (0.50 cm in a 0–10 cm VAS), physical functioning, and sleep quality
among patients with chronic pain (Wang et al., 2021). Hence, even when done properly,
the quality of the more or less same available trials can be assessed and weighted
in different ways.
Prescribing MC for patients with chronic pain is a matter of significant magnitude
for caregivers and patients on a daily basis. Clinicians around the globe often note
relief of pain and accompanying symptoms (i.e. depression, anxiety, sleep disturbances)
in patients with chronic non‐cancer pain who did not respond to established non‐pharmacological
and pharmacological therapies. Some non‐randomized prospective cohort studies have
documented such effects (Aviram et al., 2021). MC has also benefited patients with
rare painful diseases for which there will never be a RCT possible.
So what can help lost practitioners find their way out of this Bermuda Triangle? IASP
advocates primarily for well‐designed and appropriately powered future RCTs but in
the meanwhile holds a RCTs‐based ‘non‐endorsing’ position. While RCTs typically provide
the ‘state of the art’ evidence for efficacy of specific interventions, the question
is will further RCTs and subsequent SRMAs provide a solution in the case of MC? The
complex HC structure with hundreds of constituents, some of which with potential biological
activity (e.g. CBC, CBG, CBN, THCA, THCV and others), and the possible synergistic
interactions between then set significant barriers that impede the ability to conduct
traditional pharmaceutical RCTs (which are typically based on precise dosing of a
single molecule). Funding large RCTs also seems to be an ongoing challenge as indicated
by the lack of adequately powered studies published during the past five years. Furthermore,
even if the needle in a haystack is found and a certain constitutes combination does
show efficacy, the results will likely be diluted and lost in subsequent SRMAs. Are
there alternatives? GRADE (Grading of Recommendations Assessment, Development and
Evaluation) allows to include in guidelines observational studies and to increase
the level of quality of evidence in case of consistent and large effects. As mentioned
earlier, some observational studies have already been published. Additionally, MC‐registries
for chronic pain patients have been established in the meanwhile in Italy, Germany
and possibly in other countries as well.
Based on these principles, and in contrast to IASP statement, the position paper of
the European Pain Federation has recommended to consider MC as a third line therapy
for chronic neuropathic pain syndromes, whereas for all other chronic pain conditions,
the use of MC should be regarded as an individual therapeutic trial if all established
treatments have failed and after careful analyses and multidisciplinary assessment
(Häuser et al., 2018). We advocate that future practical recommendations on potential
indications, contraindications, and assessment of harms of MC should not only be based
of RCTs, but also on large, national, or even international, carefully followed and
well documented, large‐scale prospective cohorts of patients, preferably in the form
of interdisciplinary evidence‐ and consensus‐based guidelines and include patient
representatives.
Nonetheless, we wish to highlight the enormous need for rigorous MC‐related research
and for proper funding of MC studies.
CONFLICTS OF INTEREST
Elon Eisenberg received consulting fees, speaking fees, and/or honoraria from Rafa
Laboratories, Syqe medical, Medison, Teva, Pfizer. Bart Morlion is past president
of the European Pain Federation EFIC and was member of the EFIC task force which published
a position paper on cannabis‐based medicines and medical cannabis for pain management.
Over the last 5 years he received fees for service for speaker's and/or consultancy
activities from Grünenthal, Lilly, Mundipharma, Pfizer, Krka, Ache, Sandoz, Shionogi,
TEVA, GSK, Kyowa‐Kirin, Boston Scientific, Reckitt & Benckiser. Silviu Brill is Honorary
Secretary of European Pain Federation EFIC and was member of the EFIC task force,
which published a position paper on cannabis‐based medicines and medical cannabis
for pain management. He received reimbursement for travelling and accommodation by
Bioevents, a congress organiser, for co‐organising a congress on controversies on
cannabis‐based medicines in 2018 and 2019. He received consulting and speaking fees
from Rafa Laboratories, Pfizer, TEVA and Dexcell. Winfried Häuser has received reimbursement
for travelling and accommodation by Bioevents, a congress organiser, for co‐organising
a congress on controversies on cannabis‐based medicines in 2018 and 2019. He was the
head of an EFIC task force of a position paper on cannabis‐based medicines and medical
cannabis for pain management and member of a task force of a position paper on the
same topic by the German Pain Society.