Tramadol and Its Influence on Platelet Function — An Ex Vivo Study

Abstract

Tramadol, a weak μ-opioid receptor agonist and a widely used analgesic, also inhibits serotonin and norepinephrine reuptake, which could potentially influence platelet function. While the bleeding risk of selective serotonin reuptake inhibitors is well documented, the impact of tramadol on platelet aggregation, despite its widespread use, remains underexplored. Therefore, this study aims to elucidate tramadol’s effect on platelet function. This single-center laboratory study involved healthy volunteers at the Medical University of Graz, Austria. Platelet function was assessed using light transmission aggregometry following the addition of tramadol in increasing concentrations (0, 500, 1500, 4500, and 9000 ng/mL) to blood samples. Baseline and post-tramadol-addition platelet aggregation was measured using adenosine diphosphate-induced (ADP), ristocetin-induced, and thrombin-receptor activating peptide-induced (TRAM) aggregation. Statistical analysis employed the Friedman test. Seven healthy volunteers could be included in the final analysis. Platelet aggregation was assessed after ex vivo addition of tramadol (500–9000 ng/mL). No significant differences in aggregation percentages were observed between tramadol concentrations and baseline inducing activation with ADP, ristocetin, or TRAP. These findings suggest that tramadol, at therapeutic and supratherapeutic concentrations, does not significantly impair platelet function in most individuals. This supports the general safety profile of tramadol regarding platelet aggregation.

Tramadol appears safe concerning platelet function. Further research with larger cohorts is warranted to confirm these results and investigate potential interindividual variability in response to tramadol.

Keywords

Tramadol - Platelet aggregation - Analgesics - Opioids - Serotonin agents

Introduction

Tramadol, a widely used analgesic globally, is a weak μ-opioid receptor agonist positioned at the second step of the World Health Organization’s pain management ladder.[1] [2] Its primary advantage over most opioids is its milder respiratory depressant effect, even at higher doses, for example, during childbirth.[3] Although it has relatively low analgesic potency, this can be mitigated by combining tramadol with peripheral-acting agents such as nonsteroidal anti-inflammatory drugs (NSAIDs) or metamizole. Besides its opioid effects, tramadol inhibits serotonin and norepinephrine reuptake, contributing to its analgesic action by modulating descending pain pathways.[3] Tramadol has a plasma half-life of 4 hours.

In recent years, numerous analgesics have been studied for their potential impact on the coagulation cascade and platelet function. Significant changes have been documented with NSAIDs, metamizole, and paracetamol.[4] [5] [6] Additionally, co-analgesics such as selective serotonin reuptake inhibitors (SSRIs) have been linked to an increased risk of bleeding.[7] [8] [9]

While the bleeding risk associated with SSRIs due to serotonin-mediated platelet inhibition is well established, the effect of tramadol on platelet function remains inadequately explored. This is particularly relevant given tramadol’s influence on serotonin levels, a neurotransmitter that plays a crucial role in platelet activation and aggregation. Platelets possess serotonin receptors (5HT2), whose activation enhances aggregation initiated by neurotransmitters such as adenosine diphosphate (ADP) and thrombin. Moreover, serotonin-mediated vasoconstriction aids wound healing by reducing blood flow in smaller vessels.

Light transmission aggregometry (LTA) has been the reference method for measuring platelet function in clinical settings for more than 50 years. This method involves placing platelet-rich plasma (PRP) between a light source and a photocell. PRP, made by centrifuging blood, is initially cloudy and transmits little light. When agonists such as ADP or arachidonic acid are added, platelets aggregate, the PRP clears, and light transmission increases.[10] [11]

Given its widespread prescription as a pain medication, more research is urgently needed to determine tramadol’s impact on coagulation and platelet activity. This is crucial as it could have significant clinical implications, particularly in patients with an existing bleeding risk or in the perioperative settings.

The aim of this study was to investigate whether tramadol has an effect on platelet function.

Methods

Study Design and Patient Population

This study was a single-center laboratory study conducted at the Department of Anesthesiology and Intensive Care Medicine at the Medical University of Graz, Austria. Laboratory tests were carried out at the Clinical Institute for Medical and Chemical Laboratory Diagnostics at the University Hospital Graz.

Between January 2022 and July 2023, 15 healthy volunteers were asked to give 40 mL of blood each.

Ethics Approval and Consent to Participate

The study was registered at clinicaltrials.gov (NCT05237492, first registered at February 14, 2022). Approval by the Ethics Committee of the Medical University of Graz (IRB00002556) was granted on September 30, 2021 (33-553 ex 20/21). Informed consent was subsequently obtained from all subjects. All methods were performed in accordance with the relevant guidelines and regulations.

Inclusion and Exclusion Criteria

Subjects aged 18 years and older were included. Exclusion criteria were missing data regarding the primary endpoint, pregnancy, history of substance abuse, preexisting general addictive disorders, ongoing pain therapy with opioids, use of antidepressants (SNRIs or SSRIs), history of thrombocytopathy or coagulation disorders, known intolerance of opioids, and therapy with drugs that affect platelet function (acetylsalicylic acid, clopidogrel, prasugrel, ticagrelor, or similar). This information was requested by means of a questionnaire.

Measurements and Data Management

Standardized platelet function testing was performed by optical aggregometry on the Atellica COAG 360 coagulation analyzer with an integrated four-channel aggregometer (Siemens Healthineers, Vienna, Austria).

PRP was produced by centrifugation. After a baseline platelet function was measured using LTA, platelet function measurement, tramadol was added in increasing concentrations and incubated for 30 min: 0 ng/mL (baseline), 500 ng/mL in concentration group 1, 1500 ng/mL in group 2, 4500 ng/mL in group 3, 9000 ng/mL in group 4, and 0 ng/mL (repeat baseline, intended to exclude effects due to storage effects, e.g., damage, of the platelets) in group 5. Therapeutic plasma levels are 100–1,000 ng/mL.[12]

LTA was performed after the addition of 10 μM ADP, 1.2 mg/mL ristocetin, and 50 μM TRAP, respectively. Results were described as percentages of maximum aggregation.

All measurements were taken within a maximum of 3 h after the blood sample was taken.

Sample Size Calculation

At the time of the study, limited information was available regarding the potential extent of platelet inhibition caused by tramadol. To address this uncertainty, we adopted a study design modeled after previous investigations of the effects of analgesic agents on platelet function using similar measurement methods. For example, Munsterhjelm et al. studied the effects of paracetamol on platelet function in a comparably large cohort,[5] while Martini et al. examined the effects of ibuprofen on platelet function in six healthy volunteers.[13]

Based on these precedents and recognizing potential statistical limitations, we planned to include 15 subjects.

Statistical Analysis

Data were extracted from the laboratory database and merged into a database using MS Excel (Microsoft Office 2016, Redmond, WA, United States). Demographic data were presented as median (25th–75th percentile), or number (n) and percentages (%), as appropriate. Analyses were conducted using IBM SPSS statistics 27 (IBM, Redmond, WA, United States) as well as R-4.2.2, a free software environment for statistical computing and graphics (R Core Team, 2022). The significance level for all analyses was 0.05.

To compare differences in platelet aggregation between the groups (baseline, 500, 1500, 4500, 9000 ng/mL, and repeat baseline), the Friedman test was applied. To quantify the magnitude of the observed effects and guide estimation of sample sizes for future studies, Kendall’s W was calculated. Due to the exploratory nature of this analysis, no correction for multiple testing was made. In addition, an individual-case analysis was performed.

Results

Characteristics of the Study Subjects

For this part of the study, blood samples were taken from 15 healthy volunteers, of whom seven could be included in the final analysis; eight had to be excluded due to incomplete laboratory data. As a minimum requirement, at least both baseline values had to be present in order to confirm the function of the platelets by the end of the test series.

Two of the included volunteers were female (28.6%). Median age was 29.2 (28.2–33.2) years. All volunteers were of Western European origin.

Primary Outcome

Activation with ADP

Baseline platelet function without the addition of tramadol was 87.2% (85.2–93.3%). After stepwise addition of tramadol, platelet function was 84.7% (81.4–88.8%) at a tramadol concentration of 500 ng/mL, 88.4% (75.5–88.9%) at 1500 ng/mL, 84.0% (68.9–89.4%) at 4500 ng/mL, and 85.8% (84.1–88.9%) at 9000 ng/mL. At baseline (without tramadol), platelet function was 88.2% (87.3–90.8%) ([Fig. 1]).

Fig. 1 Results of all dosage groups; x-axis shows groups; y-axis shows percentage of maximal aggregation using activation with 10 μM adenosine diphosphate (ADP).

There are no significant differences between the groups (p = 0.281). Kendall’s W for ADP was 0.179.

Activation with Ristocetin

Baseline platelet function without the addition of tramadol was 89.6% (87.7–91.2%). Following the stepwise addition of tramadol, median platelet function was 88.2% (83.2–90.2%) at a concentration of 500 ng/mL, 88.5% (88.4–89.9%) at 1500 ng/mL, 88.8% (86.9–90.4%) at 4500 ng/mL, and 88.7% (88.0–89.0%) at 9000 ng/mL. In a separate baseline assessment without tramadol, median platelet function was 90.5% (87.4–92.1%) ([Fig. 2]).

Fig. 2 Results of all dosage groups; x-axis shows groups; y-axis shows percentage of maximal aggregation using activation with ristocetin 1.2 mg/mL.

There are no significant differences between the groups (p = 0.725). Kendall’s W for ristocetin was 0.081.

Activation with TRAP

Baseline platelet function without the addition of tramadol was 88.4% (88.2–89.4%). Following stepwise administration of tramadol, median platelet function was 90.1% (89.0–91.6%) at 500 ng/mL, 89.2% (88.8–89.5%) at 1500 ng/mL, 89.0% (87.5–90.3%) at 4500 ng/mL, and 90.4% (89.9–90.9%) at 9000 ng/mL. In a separate baseline measurement without tramadol, platelet function was 89.1% (88.1–90.6%) ([Fig. 3]; [Table 1]).

Fig. 3 Results of all dosage groups; x-axis shows groups; y-axis shows percentage of maximal aggregation, using activation with 50 μM thrombin-receptor activating peptide (TRAP).

Table 1 Overview of all results, values in percentage.

	0 ng/mL	500 ng/mL	1500 ng/mL	4500 ng/mL	9000 ng/mL	0 ng/mL (repeat)
ADP	87.2	84.7	88.4	84.0	85.8	88.2
Ristocetin	89.6	88.2	88.5	88.8	88.7	90.5
TRAP	88.4	90.1	89.2	89.0	90.4	89.1

Abbreviations: ADP, adenosine diphosphate; TRAOP, thrombin-receptor activating peptide.

There are no significant differences between the groups (p = 0.215). Kendall’s W for TRAP was 0.202.

Discussion

This study investigated the effect of tramadol on platelet function using an ex vivo LTA assay. Contrary to our initial hypothesis, we were unable to demonstrate a statistically significant difference in platelet aggregation between groups exposed to increasing concentrations of tramadol and the baseline group. These findings are consistent with the current state of knowledge, indicating that tramadol is not associated with an increased risk of bleeding.

Our hypothesis was based on several factors. First, tramadol’s pharmacological profile, which includes inhibition of serotonin and norepinephrine reuptake, suggested a potential influence on platelet activity. Serotonin plays a known role in platelet aggregation, and medications that affect serotonin levels, such as SSRIs have documented effects on platelet function and bleeding risk. Second, previous animal studies have indicated a potential effect of tramadol on platelet aggregation.

Casella et al. investigated the effect of tramadol on platelet aggregation in vitro in horse blood samples. Here, an effect of tramadol could be detected that is different in fasting and fed horses. Fasted horses showed a significant increase in platelet aggregation as well as an increased clot formation when tramadol was titrated. However, fasted horses showed no significant changes in platelet function compared with the control group.[14]

However, existing human data on tramadol’s effects on platelet function are limited and conflicting. Bilir et al. reported an opposite effect in an in vitro human study in women with malignant tumors, observing slowed clot formation with increasing tramadol doses using thromboelastography. Notably, clot strength, as measured by maximum clot stiffness, remained unchanged.[15] Interestingly, recent in vitro data by Iida et al. suggest that tramadol may inhibit platelet function by interfering with serotonin-mediated Rac activation and downstream MAPK signaling. However, these effects were only observed at supratherapeutic concentrations, which supports our findings that tramadol does not significantly alter platelet aggregation within the clinically relevant range.[16]

Our results show no significant overall effect of tramadol on platelet aggregation and provide indications that tramadol is generally safe for platelet function in most patients, especially compared to SSRIs.

The presence of outliers in our study raises the possibility of individual variability in response to tramadol. Unexplored cofactors, such as sex, concomitant medications, or smoking status, could influence tramadol’s effects on platelet function and warrant further investigation. However, caution should be exercised in perioperative settings, particularly in high-bleeding-risk procedures like vitreoretinal surgery or neurosurgical operations, or in patients with preexisting bleeding diatheses.

The small initial sample size (n = 15) and subsequent exclusions due to incomplete data (n = 8) underscore the importance of rigorous data collection and quality control in medical research. Larger, more comprehensive studies are needed to confirm our findings, explore potential influencing factors, and definitively characterize the relationship between tramadol and platelet function in humans. By reporting effect sizes, our findings contribute to a better understanding of tramadol’s impact on platelet function and help ensure that subsequent research is appropriately powered and methodologically robust.

Limitations

As the study design was based on similar studies, the number of cases was set to be very small. Also the study design as an ex vivo laboratory study could lead to possible damage to the platelets or changed effects of tramadol on platelets itself. Another limiting factor is certainly the unexpectedly high number of exclusions due to incomplete or missing data.

Conclusions

In this ex vivo laboratory study, no significant effect of tramadol on platelet function, even at concentrations far above the target plasma concentrations, could be demonstrated.

Contributorsʼ Statement

P.Z.: Conceptualization, data curation, formal analysis, methodology, writing—original draft preparation. H.B.-C.: Conceptualization, data curation, formal analysis, methodology, writing—original draft preparation. F.P.: Conceptualization, data curation, formal analysis, methodology, writing—original draft preparation. G.H.: Data curation, formal analysis, writing—original draft preparation. M.E.: Validation, formal analysis, writing—reviewing and editing. M.E.: Validation, formal analysis, writing—reviewing and editing. L.H.: Validation, formal analysis, writing—reviewing and editing. N.S.: Validation, formal analysis, writing—reviewing and editing. G.R.-S.: Conceptualization, validation, formal analysis, writing—reviewing and editing. P.Z.: Validation, formal analysis, writing—reviewing and editing.

Acknowledgment

We want to thank all the volunteers who gave their blood and the whole anesthesiology and laboratory team at the Medical University Graz for the support.

Data Sharing Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Publication History

Received: 12 May 2025

Accepted after revision: 30 December 2025

Article published online:
12 February 2026