In a study of 70 products, the Consumer Association of North Rhine-Westphalia said it found everything from weight loss powders to products promising to increase libido were dangerous.
It also found some unscrupulous vendors were using fraudulent stamps of approval from pharmacies or legitimate organisations such as Stiftung Warentest, which rates products for their effectiveness.
The Consumer Association recommended consumers only buy supplements certified by German authorities or carefully research any foreign products before they make a purchase.
Among the most dangerous goods were products meant to improve people's sexual potency, some of which contained tadalafil. The ingredient can be found in legitimate drugs used to treat sexual dysfunction, but overdoses can be fatal.
Sibutramine, which has been officially withdrawn from the German market due to fears it increases the risk of stroke, was found in some diet pills. Other slimming concoctions contained the potentially carcinogenic substance phenolphthalein.
The report also said that unscrupulous vendors had been using slack regulation of the internet in order to get banned substances onto the German market.
Almost none of the products would be allowed to be sold in German pharmacies due to basic deficiencies, such as improper labelling, the study found.