The Medicines and Healthcare products Regulatory Agency said some batches of Sanofi-Aventis' enoxaparin (Clexane) contain low levels of an impurity called over-sulphated chondroitin sulphate.
The agency pointed out that there was no evidence that this was linked to risks to patients and advised that patients should continue to receive the drug as normal. But if patients are worried they should talk to a health professional.
In 11 other countries there has been an increase in ADRs with other heparin products.
This has not been the case in the UK, said the MHRA but nurses and medics should be vigilant to possible reactions.
Sanofi Aventis reported that no more enoxaparin batches manufactured since 1 April 2008 were affected and should be in the supply chain from June 2008.