The Orange Order has written to NI Secretary Brandon Lewis to express concern over the impact the Northern Ireland Protocol could have on medicine supplies.
Grand Master Edward Stevenson called on the UK Government to ensure that people in Northern Ireland would not be negatively affected.
Under the terms of the protocol Northern Ireland will be aligned with the EU's rules on medicines from January 2022.
This will affect a number of areas, including vaccinations. Northern Ireland is currently part of the UK's coronavirus vaccine rollout due to a grace period which will expire at the end of this year.
As a result of the protocol, medicines for Great Britain and Northern Ireland are regulated under different regimes but overseen by the UK's Medicines and Healthcare Products Regulatory Agency.
The Orange Order is the latest unionist group to express their displeasure at the protocol, agreed post-Brexit to prevent a hard border on the island of Ireland by keeping Northern Ireland aligned with EU trade rules.
Unionists feel this has left them cut off from the rest of the UK, while a number of issues have emerged as a result of protocol, with businesses reporting difficulties importing goods from Great Britain.
In his letter Grand Master Stevenson expressed concern that Northern Ireland remains under the EU regulatory regime for many products, including medicines.
Among the concerns raised were that when the grace period expires there is no clarity as to what effect it will have on the regulatory process or supply chain logistics.
"This divergence will necessitate different packaging for GB goods entering NI and may well result in a reduction of products made available to the NI market," an Orange Order statement said.
"There is a very real concern that drugs entering Northern Ireland from Great Britain which in future may not be compliant with EU rules, will be prevented from being supplied to NI patients."
Mr Stevenson also highlighted the recent issues around the EU's Covid-19 vaccination rollout as one potential area that Northern Ireland could lose out.
"The need for EU approval means that NI will be out of step with GB. It is worth noting that the EU failed to approve the Covid-19 vaccines as quickly as the UK, (including what has now been shown to be unfounded concerns about the Oxford vaccine) and therefore the UK rollout programme has been excellent whereas the success of the EU vaccination rollout remains much lower," the statement said.
The Orange Order called "on the UK Government to take decisive steps to provide assurances that citizens in Northern Ireland will continue to be able to avail of the full range of clinical treatments currently available to citizens in the rest of the United Kingdom".
TUV leader Jim Allister has also expressed concern about Northern Ireland being tied to the EU on medicines.
“Gone are the days when people used to argue that the protocol would result in us having the best of both worlds," the North Antrim MLA said.
“We now need to face up to the fact that the protocol would be, if fully operational as it is set to be next year, quite literally bad for your health.”