Do Steroids Make Your Blood Thicker or Thinner?

Amateur lifters hear two stories about steroids and blood clots. One says any cycle makes your blood "sticky." The other says clotting labs look fine, so the risk is imaginary. A 2023 analysis from the HAARLEM cohort — 99 men who started their own cycles and were followed with repeated clotting panels — shows a more boring middle: many clotting proteins rose during use, some anticoagulant markers rose too, thrombin generation went down, and the overall clotting-factor balance did not look like a clear procoagulant state. Three months after coming off, the panel was back to baseline.

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1. What this paper is

Camilleri, Smit, van Rein and colleagues used the HAARLEM study (Health risks of Anabolic Androgenic steRoid use by maLE aMateur athletes). One hundred men aged 18 and up who planned to start a self-directed AAS cycle were enrolled between 2015 and 2018 in the Netherlands. Ninety-nine were analyzed for this coagulation paper (one man on anticoagulants was excluded).

They measured clotting factors, free protein S, von Willebrand factor antigen, D-dimer, clot lysis time and thrombin-generation parameters at baseline, in the last week of the cycle, three months after stopping, one year after inclusion, and (for most markers) two years out. Cycles were the athletes' own: median 13 weeks, median about 901 mg per week across a median of four different androgens. The drugs were not supplied by the investigators.