Because the United States does not regulate drug pricing or negotiate prices nationally like other countries, generic medicines have long been a safety valve for American patients, allowing them to obtain needed medicines at lower costs. Brand name medicines are granted patent protection for a number of years after they enter the market. Historically, after the patent expires, generic copies have entered the fray, bringing prices down, often sharply.

But that pattern is changing, researchers and policy makers say.

The cost of many generic medications has increased so much over the past year that prices for many common generic drugs in the United States have surpassed those of their brand-name equivalents in other developed countries, a new analysis by the website Pharmacychecker, which guides patients in the mail order purchase of medications, has found.

For example, a 90-day supply of the generic heart medicine digoxin sells for $187 in New York; the branded version, Lanoxin, sells for $24.30 in Canada. A month’s supply of a generic steroid to treat inflammatory bowel disease sells for $1,625 in the United States, while the branded version sells for $155.70 in Canada. A three-month supply of the generic cholesterol lowering drug pravastatin costs $230 in this country, but $31.50 for the branded drug Pravachol in Canada.

Senator Klobuchar, a Minnesota Democrat, said the time was right for legislation as drug costs have risen, while salaries have not. “The economy has stabilized but it’s really hard for the middle class to get by, and this is a glaring example of where we are so out of line with other countries,” she said in a phone interview. “One solution is to let some competition in from over the border.”

There are many reasons that generic prices may fluctuate. The price of a key ingredient may increase markedly, or competition may decrease as manufacturers leave a market, leading to price rises.