Willow does decrease platelet aggregation and therefore anyone on anticoagulants or with a bleeding disorder should not use Willow unless under the guidance of a professional. The anti-thrombotic effect is considered mild and less than that of aspirin.
Here are a couple of research abstracts that help to answer this question.
Zhong Yao Cai. 1999 Mar;22(3):131-3.
[Isolation of resisting thrombus and arteriosclerosis compounds in leaves of Salix matsudana].
[Article in Chinese]
In this paper, three compounds were isolated and identified from the leaves of Salix matsudana. They are apigenin-7-0-beta-D-glucopyranside(I), luteolin-7-0-beta-D-glucopyranside(II), compound III. Compound I and II are isolated firstly from Salix spp., compound III is found firstly in the world. Furthermore, study on effect of arachidonic acid metabolisin in rat platelets by them with radio-chromatography found that they can significantly inhibit the production of 12-HETE(12-hydroxy-5,8, 10,14-eicosatetraenoic acid), which can induce allergy and arteriosclerosis. The production of apigenin-7-0-beta-D-glucopyranside being hydrolyzed was apigenin, it can inhibit TXB2(thromoxane B2) which can induce platelet aggregation.
- [PubMed - indexed for MEDLINE]
Planta Med. 2001 Apr;67(3):209-12.
Effect of salicis cortex extract on human platelet aggregation.
The bark of Salix species contains several prodrugs of salicylate, mainly salicin. The aim of this study was to investigate if during pain treatment with Salicis cortex extract platelet aggregation was affected. A total of 51 patients were enrolled in the study. Thirty-five patients suffering from acute exacerbations of chronic low back pain received randomly and double-blind either Salicis cortex extract with 240 mg salicin/day (n = 19) or placebo (n = 16). Further sixteen patients with stable chronic ischemic heart disease were given 100 mg acetylsalicylate per day. Platelet aggregation was studied using an aggregometer. As aggregating agents, arachidonic acid (500 micrograms/ml), adenosine di-phosphate (2 x 10(-5) M) and collagen (0.18 microgram/ml) were used. The mean maximal arachidonic acid induced platelet aggregation was 61%, 78% and 13% in the Salicis cortex extract, placebo and acetylsalicylate groups. Acetylsalicylate had a significant inhibitory effect on platelet aggregation compared to Salicis cortex extract (p = 0.001) and placebo (p = 0.001). There was also a significant difference between the placebo and the willow bark-treated groups in the maximal plateletaggregation induced by arachidonic acid (p = 0.04) and ADP (p = 0.01). No statistical difference was found between the groups when collagen was applied to the human platelets. Daily consumption of Salicis cortex extract with 240 mg salicin per day affects plateletaggregation to a far lesser extent than acetylsalicylate. Further investigation needs to clarify if this finding is of clinical relevance in patients with impaired thrombocyte function.