A survey of over 10,000 patients by the MS Society found access to drugs was 68% in Northern Ireland, 40% in England, 36% in Scotland and 30% in Wales.
The charity said that limited patient access to specialists was the main reason for the prevalence of under-treatment.
Only two other EU countries, Poland and Romania, had a lower level of uptake of medication for the disease.
The relatively high level of medication for MS in Northern Ireland reflects the fact that only that country offers patients a twice-yearly review with a specialist.
The MS Society called for the NHS to provide every MS patient with a personalised care plan.
The study found that 41% of patients who said they had enough information about drugs still did not take one.
It concluded: “This could be due to barriers to accessing medicines; because individuals make an informed decision not to take them; or because they don’t know what information is out there, such as around new treatments or new evidence of efficacy.”
Nick Rijke, Director for Policy and Research at the MS Society, commented: “These findings worryingly suggest that the likelihood of someone receiving a life-changing treatment is often based on luck – like where they live or how helpful their healthcare professional is.”
The Society noted that NHS rationing was denying many patients access to the more expensive MS drugs, even though these might be more effective.
MS affects about 100,000 people in the UK, three quarters of them female.