The Bern-based organization said people who buy slimming products online are putting their health at risk.
Of 61 products it tested, more than two-thirds contained active ingredients that were not declared, Swissmedic said.
More than half of these products were also found to contain sibutramine, a drug that was withdrawn from the market in 2010 after it was associated with increased cardiovascular problems and strokes.
Most of the remedies analyzed by Swissmedic originated from Southeast Asia.
The agency said that of 41 products advertised as natural or herbal, 35 contained dangerous chemicals.
Ten products were coffee-based drinks — some with harmful substances added — and eight contained synthetic ingredients.
“Experience has shown that demand for slimming aids increases in the spring,” Swissmedic said.
“Many people order such preparations on the internet without knowing what they are actually going to receive.”
Swissmedic said the results of its testing showed that products advertised as slimming aids or medicines from uncontrolled sources “can represent a huge health risk”.
It cited reports from other countries of consumers were sent to hospital after taking products containing sibutramine.
The agency urged people not to buy and consume medicines or nutritional supplements from “dubious suppliers on the internet”.
It added that around 90 percent of suppliers who claim to be reputable internet pharmacies “operate illegally and without official approval or monitoring”.