Adverse reactions of dimethyl sulfoxide in humans: a systematic review

Background: Dimethyl sulfoxide (DMSO) has been used for medical treatment and as a pharmacological agent in humans since the 1960s. Today, DMSO is used mostly for cryopreservation of stem cells, treatment of interstitial cystitis, and as a penetrating vehicle for various drugs. Many adverse reactions have been described in relation to the use of DMSO, but to our knowledge, no overview of the existing literature has been made. Our aim was to conduct a systematic review describing the adverse reactions observed in humans in relation to the use of DMSO. Methods: This systematic review was reported according to the PRISMA-harms (Preferred Reporting Items for Systematic reviews and Meta-Analysis) guidelines. The primary outcome was any adverse reactions occurring in humans in relation to the use of DMSO. We included all original studies that reported adverse events due to the administration of DMSO, and that had a population of five or more. Results: We included a total of 109 studies. Gastrointestinal and skin reactions were the commonest reported adverse reactions to DMSO. Most reactions were transient without need for intervention. A relationship between the dose of DMSO given and the occurrence of adverse reactions was seen. Conclusions: DMSO may cause a variety of adverse reactions that are mostly transient and mild. The dose of DMSO plays an important role in the occurrence of adverse reactions. DMSO seems to be safe to use in small doses. Registration: PROSPERO CRD42018096117.


Introduction
The first medical report on the use of dimethyl sulfoxide (DMSO) as a pharmacological agent was published in 1964 1 . A year later, the use of DMSO in humans was terminated because experimental studies had shown refractive index changes to the lens of the eye 1,2 . Years later, DMSO was again approved for use in humans since this side effect was only proven in animal studies 2 . DMSO has since been used for a variety of purposes, such as treatment of musculoskeletal and dermatological diseases, cryopreservation of stem cells, treatment of interstitial cystitis, treatment of increased intracranial pressure, and many more 3-9 .
DMSO is a colourless liquid, which is rapidly absorbed when administered dermally or orally 10,11 . DMSO is used as a cryoprotectant because it decreases osmotic stress and cellular dehydration, and thereby enables stem cells to be stored for several years 12 . DMSO is mostly excreted through the kidneys, but a small part is excreted through the lungs and liver 10 . Part of the DMSO is transformed to the volatile metabolite dimethyl sulfide, which gives a characteristic garlic-or oyster-like smell when excreted through the lungs 10 . DMSO may induce histamine release, which can be the reason for adverse reactions such as flushing, dyspnoea, abdominal cramps, and cardiovascular reactions 11 .
To our knowledge, no systematic reviews have been performed on the adverse reactions of DMSO. Our aim was therefore to provide an extensive overview of the suspected adverse reactions to DMSO in humans.

Protocol and eligibility criteria
Our study-protocol is registered at PROSPERO (Registration number: CRD42018096117). The systematic review was performed according to PRISMA-harms (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines 13 .
No limitations were set on the date of publication. The language was restricted to English, Danish, Swedish, Norwegian, and Russian. We included all original studies that administered DMSO to humans and included five or more participants. There was no gender or age restriction. For a study to be included, the authors had to suspect that an observed adverse reaction could be caused by DMSO.

Primary outcome
The primary outcome was any adverse reaction seen in relation to the use of DMSO in humans.

Literature search
The search was performed in PubMed (1966-present), EMBASE (1980-present), and the Cochrane Library. The databases were last searched on February 23, 2018. Our search strategy was formulated with the help of a medical research librarian.
The search string used in PubMed was: ((dimethyl sulfoxide) OR DMSO) AND ((((((administration and dosage) OR adverse reactions) OR alternate effects) OR secondary response) OR toxicology) OR side effects)). The search was restricted to humans. The search string was adapted to EMBASE and Cochrane Library using the same search-words as abovementioned.
The search string used in EMBASE was: ((dmso or dimethyl sulfoxide) and ((side effect or toxicology or secondary response or alternate effects or alternate reactions or (administration and dosage)) and (dmso or dimethyl sulfoxide))).mp. The search was restricted to humans, articles and Medline journals were excluded.
The search string used in Cochrane was: (adverse drug events and dimethyl sulfoxide). The search was restricted to trials.

Study selection and data extraction
Two authors (B.K.M. and D.Z.) independently screened title and abstract according to the eligibility criteria using www. covidence.org. Discrepancies were resolved by discussion. One author screened the full-text articles (B.K.M.). Russian articles were screened by an author fluent in Russian (M.H.). If M.H. was in doubt regarding inclusion of a study the results were presented to B.K.M. and then discussed until a mutual decision was made.
After the screening process was finished, all included studies were imported to an Excel sheet (Microsoft Excel 2016). Data extraction was performed by two authors (M.H. extracted from the Russian articles and B.K.M. extracted from the rest). Data extracted were: author, publication year, country, study characteristics (study design, sample size, size of comparison group if present, time to follow-up), use of DMSO (reason for use, treatment duration, administration route, dose of DMSO), and adverse reactions observed (number of persons experiencing an adverse reaction, method of assessing, and duration of adverse reaction).

Analysis
The Newcastle-Ottawa-Scale was used to assess the risk of bias in non-randomized observational studies 14 . Risk of bias in randomized controlled trials was assessed using the Cochrane Handbook "Risk of Bias" assessment tool 15 . Risk of bias was assessed at the outcome level.
The primary summary measure was percentage of persons experiencing an adverse reaction, as well as the range in which a reaction occurred in the studies included. No meta-analysis and further summery measures were planned due to the expected large heterogeneity of the studies.

Amendments from Version 1
In this updated version an additional table (Table 9)
The most commonly reported gastrointestinal adverse reactions were nausea and vomiting. The incidence of nausea seems to be less common with the transdermal administration of DMSO compared with intravenous administration. The majority of studies reported an incidence of nausea between 2-14%, with the exception of one study, reporting an incidence of 32% 2 . In one study that failed to specify the dose, 8 of 42 patients reported nausea and anorexia, but the symptoms disappeared in five of the eight patients when the dose of DMSO was reduced 45 .
Often the studies had short follow-up periods (less than 24 hours), especially when DMSO was used as a cryoprotectant. The study reporting the highest incidence of nausea had a follow-up period of 5 days 48 , and the authors concluded that the high incidence (min%-max%) are the lowest and highest observed incidence of an adverse reaction observed in the group of studies included. ‡ Nausea and vomiting are reported as one combined adverse reaction in some studies. of nausea observed might be due to the long follow-up period 48 . In another article using the same data 119 , it was suggested that the delayed nausea was due to gastrointestinal mucosal damage, and only the initial nausea could be related to DMSO, and therefore we decided only to include the data from the first 2 days after infusion 48  . In five studies, patients discontinued treatment due to halitosis 9,45,83,94 . In five studies, all patients experienced halitosis 9,45,83,94 . Unlike halitosis, other gastrointestinal side effects were reported more often when DMSO was administered intravenously, than transdermally or intravesically.
One study reported a severe case of nausea, vomiting, and abdominal cramps in one patient with an acute allergic reaction 59 . However, in most studies the reported gastrointestinal reactions were transient and mild, often lasting only minutes to a couple of hours 16 , and one was a preliminary report 91 . Except for one study 66 , all studies reporting cardiovascular and respiratory reactions administered DMSO intravenously (Table 3 and  Table 4).
Bradycardia was defined as a heart rate less than 60 beats per minute 41,61 and was often transient 23,61,90,115 , but cases where atropine was needed are described 49,96 . A lowered heart rate not enough to be considered bradycardia was observed in four studies 39,41,54,61 .
In some studies, hypertension did not require intervention 61,102 , but cases where medication was needed to control the hypertension, or where treatment was stopped due to hypertension, are described 41,54,85 . Hypotension was also described as transient most of the time 18,23,68,87,104 , with some cases needing intervention 40,51,54 .
One study reported 11 cases of transient extrasystoles in 22 patients receiving cryopreserved autologous blood stem cells, monitored with Holter during infusion 73 . There were two studies reporting cases of asystole during embolization of dural arteriovenous fistulas with a substance called Onyx, a nonadhesive liquid embolic agent dissolved in DMSO 91,100 .

Neurological reactions
Headache is the most common neurological adverse reaction reported. In one study, headache was the reason for withdrawal of 2 out of 21 patients being treated with DMSO 116 .
Three studies using DMSO as a cryoprotectant in stem cell transfusions described seizures after administration 18,36,47 . Severe encephalopathy was observed in one patient 99 , and transient cranial nerve III and IV palsy was observed in one patient after Onyx embolization 34 . One study described neurological symptoms occurring during and after transfusion, but they did not define neurological symptoms in detail 86 .

Urogenital reactions
Few urogenital reactions were described (Table 6 and Table 7). Hemoglobinuria was described as an adverse reaction seen after transfusion of stem cell products 39,51,56,73 . However, hemoglobinuria is often attributed to erythrocyte debris in the transplant material and has thus not been interpreted as being caused by DMSO 39,73 . The other urogenital reactions (Table 6 and  Table 7) all occurred after DMSO instillation in the bladder 38,49,97 .

Other reactions
Only one study in this review administered DMSO as eyedrops 114 . In this study, two patients experienced severe conjunctival hyperemia due to allergic reactions, and 25% of patients experienced a stinging sensation when eye-drops were applied 114 . Other studies performed eye examinations to determine whether DMSO caused changes in the lens; however, no such cases were observed 2,45 .
Hyponatremia occurred in six patients after they received large doses of DMSO as treatment for cranial hypertension 62 . This adverse reaction was not reported in other studies (Table 8).
Very few cases of serious adverse reactions associated with DMSO have been described 18,36,51,59 .
Overall, most studies administered DMSO intravenously or transdermally (Table 9) Risk of bias within studies In this review, we included 76 cohort studies, of which 64 were prospective 2 Using this scale, studies were given zero to nine stars. A high number of stars equals low risk of bias and vice versa. The studies in this review had a median value of 5 stars, with a range of 2-8. No studies received the highest possible value of nine stars. Very few studies had a comparison group that did not receive DMSO, and often the occurrence of adverse reactions was poorly described. There were 24 randomized controlled trials (Figure 2). Many studies received an unclear risk of bias because often it was vaguely described how adverse reactions were reported.      Overall, there was a high risk of bias when assessing the description of adverse reactions. Some studies were not assessed for bias due to being case-reports, preliminary trials, or because they included more than one study design 17,19,28,62,84,91,99,109,111,113 .

Discussion
Gastrointestinal and dermatological adverse reactions were the most commonly reported in the included studies. Cardiac adverse reactions only occurred when DMSO was administered intravenously, whereas dermatological reactions mostly occurred when DMSO was administered on the skin. Serious neurological and cardiac reactions were rare and only described in few studies. There seems to be a dose-response relationship between DMSO and adverse reactions with no or mild reactions in low doses.
Many studies on the use of DMSO have been performed in Russia. These studies have not been readily accessible to the global community due to the language barrier. In this review, we have included not only studies dating back almost 50 years, but also articles written in Russian, which is an important strength of the review. This study has several limitations: 1) Some studies used the NCI-CTC (National Cancer Institute's Common Terminology Criteria for adverse events), but often no scale was used, and the occurrence of adverse reactions were poorly reported.
2) It was difficult to make conclusions on the frequency of a specific adverse reaction, because the exact number of patients experiencing a reaction was often not stated. 3) Several studies using DMSO as a cryoprotectant concluded that other factors affected the occurrence of adverse reactions 7,85,86 . One study prospectively looked at the adverse reactions observed in relation to autologous transplantation in 64 European Blood and Marrow Transplant Group centers 7 . They had difficulties isolating the effects of DMSO from confounding factors such as cell breakdown products and conditioning chemotherapy. Factors such as age, gender, volume transfused, granulocyte concentration, clumping of transplant material, and amount of red blood cells played a role in the occurrence of adverse reactions 61,86,120-122 . Another study believed that acute volume expansion, electrolyte imbalance and vagal responses to the coldness of the freshly thawed infusate were more likely reasons for cardiac arrhythmias during stem cell transfusions than the DMSO infused 123 . This differs from other studies, which found a clear connection between dose of DMSO and occurrence of cardiac adverse reactions 41,67,71,75,78,85,86,93,101,115 . Therefore, it is possible that some adverse reactions are more or less common than found in this review. The rarer side effects are often reported in case reports, which often did not meet the eligibility criteria in this review. However, we have included several larger studies in this review, and they found a very small occurrence of serious adverse events 7,55,66,74 .
In conclusion, adverse reactions due to DMSO are often mild and transient and do not qualify as serious adverse events. Cardiovascular and respiratory adverse reactions occur mostly when DMSO is administered intravenously, whereas dermatological reactions have a higher incidence when DMSO is administered transdermally. An important finding is that the occurrence of adverse reactions seems to be related to the dose of DMSO, and it therefore seems safe to continue the use of DMSO in small doses.

Data availability
All data underlying the results are available as part of the article and no additional source data are required.

Grant information
The author(s) declared that no grants were involved in supporting this work.

Introduction
The term "possible adverse reactions" is incorrect. Suspected adverse reactions is more reasonable Methods If Russian articles were screened by only one author, how were discrepancies resolved in these cases? Specify which authors extracted the data, and whether this was done independently.

Results
The term "possibly due" is incorrect. There are 4 levels in describing associations between medicines and suspected adverse reactions. The authors should revise their terminology. You state "in some studies patients discontinued treatments due to halitosis"; however, you have provided references for 5 studies -the report can be more precise.

Discussion
How does "including Russian studies" strengthen the review? What about several other languages that have been omitted? You state that there seems to be a dose-response relationship, and have drawn similar conclusions. However, at no point in the results do you report data to support this claim. You state that studies reported associations between dose and the occurrence of adverse reactions, but fail to report the doses in question. Please enumerate the limitations of your review.

Are the rationale for, and objectives of, the Systematic Review clearly stated? Yes
Are sufficient details of the methods and analysis provided to allow replication by others? Partly

Is the statistical analysis and its interpretation appropriate? Partly
Are the conclusions drawn adequately supported by the results presented in the review?

Are the conclusions drawn adequately supported by the results presented in the review? Partly
No competing interests were disclosed.

Competing Interests:
Reviewer Expertise: Adverse drug reactions; systematic reviews I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.
Author Response 31 Jul 2019 , Herlev Hospital, Herlev, Denmark bennedikte Madsen Dear Igho J. Onakpoya, Thank your for reviewing our manuscript "Adverse reactions of dimethyl sulfoxide in humans: a systematic review". Your comments were very helpful and we appreciate the effort you put in to reviewing our manuscript. We have addressed the individual questions in the section below. We hope your find our replies satisfactory. Questions are written in and answers in plain. italic Q1: The term "possible adverse reactions" is incorrect. Suspected adverse reactions is more reasonable. A1: We have changed the paragraph in the introduction section to "suspected adverse reactions". Q3: The term "possibly due" is incorrect. There are 4 levels in describing associations between . medicines and suspected adverse reactions. The authors should revise their terminology A3: We have rewritten the paragraph so it now states: "Gastrointestinal adverse reactions were reported in 61 studies. Of these, 10 studies were randomized controlled trials." Q4: You state "in some studies patients discontinued treatments due to halitosis"; however, you have provided references for 5 studies -the report can be more precise A4: We have made our report more precise and it now states: "In five studies, patients discontinued treatment due to halitosis."

Q5: How does "including Russian studies" strengthen the review? What about several other
? languages that have been omitted A5: A Russian Chemist, Dr. Alexander Saytzeff, identified DMSO in 1866, however it was not used for medical use at the time . But the fact that he was Russian might have been the reason why Russian scientists have made numerous studies using DMSO. We therefore thought it would be 1 Russian scientists have made numerous studies using DMSO. We therefore thought it would be valuable to include Russian articles since many of these studies have never been translated, and therefore are not available to the international scientific society. Of course, we could have included many other languages, but we thought it most relevant to include Russian since we observed a large amount of articles in Russian during our initial examination of the subject.
Q6: You state that there seems to be a dose-response relationship and have drawn similar conclusions. However, at no point in the results do you report data to support this claim. You state that studies reported associations between dose and the occurrence of adverse reactions but fail to report the doses in question. A6: As mentioned in our study several studies described a dose-response relationship between the amount of DMSO and the occurrence of adverse reactions ( ). However, since the doses of DMSO and the route of administration differ between the use of DMSO are not clear. This also means there are side effects from the drug or treatment facilitated by the DMSO; is it possible to separate these effects in any way? Do the authors of the many papers selected for analysis recommend an upper limit to the amount of DMSO given or have a strategy for minimising the dose?
In their final paragraph the authors suggest that "reactions due to DMSO are often mild and transient". In their previous paragraph they admit that the case reports the less common and more severe side effects which did not meet the eligibility criteria of this review. However as long ago as 2005 it was possible to identify severe side effects in an appreciable number of cases (Windrum , 2005 ). Furthermore et al. although they do not separate the factors responsible the authors of reference 7 record a SAE (Grades 3, 4 and 5) profile in excess of 3%. The authors should possibly be a little more circumspect in this paragraph particularly as they recommend the use of DMSO in (unspecified) small doses. Q1: It is not easy to ascertain the number of patients receiving DMSO intravenously and those receiving it by other routes. A small table could clarify this. A1: We have added a table (Table 9) to our manuscript describing the route of administration of DMSO.
Q2 & 3: There seems to have been no attempt to quantify the dose of DMSO which patients have received or to characterize the severity of the reactions and relate these. Furthermore DMSO is usually a vehicle to facilitate giving the patient some other treatment e.g. a stem cell transplant or drug so the reasons for the use of DMSO are not clear. This also means there are side effects from the drug or treatment facilitated by the DMSO; is it possible to separate these effects in any way? Do the authors of the many papers selected for analysis recommend an upper limit to the amount ? of DMSO given or have a strategy for minimising the dose In their final paragraph the authors suggest that "reactions due to DMSO are often mild and transient". In their previous paragraph they admit that the case reports the less common and more severe side effects which did not meet the eligibility criteria of this review. However as long ago as 2005 it was possible to identify severe side effects in an appreciable number of cases (Windrum et al., 2005 ). Furthermore although they do not separate the factors responsible the authors of reference 7 record a SAE (Grades 3, 4 and 5) profile in excess of 3%. The authors should possibly be a little more circumspect in this paragraph particularly as they recommend the use of DMSO in . (unspecified) small doses A2 & 3: DMSO is most often used as a vehicle in combination with other drugs. Therefore, it is not possible to separate completely the adverse reactions related to the use of DMSO and the adverse reactions related to other drugs, since adverse reactions such as nausea, vomiting, headache etc. are not specific for solely DMSO. As described by the authors of reference 7 , it was difficult to isolate the effect of DMSO from side effects related to conditioning chemotherapy. The only adverse effect that can solely be attributed to DMSO is halitosis. Therefore, we could not conclude that DMSO was the cause of SAE's in reference 7 and have not included it in our study. Correctly, Windrum et al. describes several adverse reactions which may be contributed to DMSO. However, the study does not describe the seriousness of the adverse reactions. As described in our study, it is very possible that some events are underrepresented in our study, which is a limitation. The upper limit was not described by any studies; on the other hand several studies evaluated different doses of DMSO and found that a lesser amount of DMSO created fewer adverse reactions ( .). Based on this observation, we feel confident that the use of small amounts of DMSO is recommendable, since DMSO works well as a vehicle. However, limiting the amount would always be desirable.
No competing interests were disclosed. Competing Interests: