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Pharmacokinetics of inorganic cobalt and a vitamin B12 supplement in the thoroughbred horse: differentiating cobalt abuse from supplementation

Hillyer, Lynn; Ridd, Z.; Fenwick, S.; Hincks, Pamela; Paine, Stuart


Lynn Hillyer

Z. Ridd

S. Fenwick

Pamela Hincks

Professor of Pharmacometrics



While cobalt is an essential micronutrient for vitamin B12 synthesis in the horse, at supraphysiological concentrations, it has been shown to enhance performance in human subjects and rats, and there is evidence that its administration in high doses to horses poses a welfare threat. Animal sport regulators currently control cobalt abuse via international race day thresholds, but this work was initiated to explore means of potentially adding to application of those thresholds since cobalt may be present in physiological concentrations.

To devise a scientific basis for differentiation between presence of cobalt from bona fide supplementation and cobalt doping through the use of ratios.
Study design

Six Thoroughbred horses were given 10 mL vitamin B12/cobalt supplement (Hemo‐15®; Vetoquinol, Buckingham, Buckinghamshire, UK., 1.5 mg B12, 7 mg cobalt gluconate = 983 μg total Co) as an i.v. bolus then an i.v. infusion (15 min) of 100 mg cobalt chloride (45.39 mg Co) 6 weeks later. Pre‐and post‐administration plasma and urine samples were analysed for cobalt and vitamin B12.

Urine and plasma samples were analysed for vitamin B12 using an immunoassay and cobalt concentrations were measured via ICP‐MS. Baseline concentrations of cobalt in urine and plasma for each horse were subtracted from their cobalt concentrations post‐administration for the PK analysis. Compartmental analysis was used for the determination of plasma PK parameters for cobalt using commercially available software.

On administration of a vitamin B12/cobalt supplement, the ratio of cobalt to vitamin B12 in plasma rapidly increased to approximately 3 and then rapidly declined below a ratio of 1 and then back to near baseline over the next week. On administration of 100 mg cobalt chloride, the ratio initially exceeded 10 in plasma and then declined with the lower 95% confidence interval remaining above a ratio of 1 for 7 days. For two horses with extended sampling, the plasma ratio remained above one for approximately 28 days after cobalt chloride administration. The effect of the administration of the vitamin B12/cobalt supplement on the urine ratio was transient and reached a peak value of 10 which then rapidly declined. However, a urine ratio of 10 was exceeded, with the lower 95% confidence interval remaining above a ratio of 10 for 7 days after cobalt chloride administration. For the two horses with extended sampling, the urine ratio remained above 10 for about 18 days (442 h) after cobalt chloride administration even though the absolute cobalt urine concentration had dropped below the international threshold of 100 ng/mL after 96 h.
Main limitations

Only one vitamin B12/cobalt product was evaluated, a limited number of horses were included, the horses were not in full race training and the results may be specific to this population of horses.

The results provide the basis for a potential strategy for allowing supplementation with vitamin B12 products, while controlling the misuse of high doses of cobalt, through a combination of international thresholds and ratios of cobalt to vitamin B12, in plasma and urine.

Journal Article Type Article
Acceptance Date Sep 8, 2017
Online Publication Date Oct 20, 2017
Publication Date May 1, 2018
Deposit Date Apr 18, 2018
Publicly Available Date Oct 21, 2018
Journal Equine Veterinary Journal
Print ISSN 0425-1644
Electronic ISSN 2042-3306
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 50
Issue 3
Pages 343-349
Keywords horse; cobalt; vitamin B 12; threshold; regulatory
Public URL
Publisher URL
Additional Information This is the pre-peer reviewed version of the following article: Hillyer, L. L., Ridd, Z. , Fenwick, S. , Hincks, P. and Paine, S. W. (2018), Pharmacokinetics of inorganic cobalt and a vitamin B12 supplement in the Thoroughbred horse: Differentiating cobalt abuse from supplementation. Equine Vet J, 50: 343-349, which has been published in final form at This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.


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