Judges had to rule on the substances after a drug dealer appealed a two-years suspended sentence for selling herbs mixed with artificial cannabinoids - the active ingredients of cannabis - over the internet.
The Bavarian state court in Landshut found that the man knew that he was selling consciousness-altering substances to people inside and outside Germany.
His sales were often in amounts significantly larger than the 1.75 grammes the state court decided would be "minor" for sentencing purposes after hearing evidence from experts.
The "minor" threshold is important because people convicted of dealing drugs over that amount must by law be handed a minimum sentence of at least one year – or two when convicted of importing drugs.
Now the Supreme Court has ruled that for two of the synthetic cannabinoids the "minor" threshold would be two grammes, while two others would have a "minor" amount of up to six grammes.
A "minor" amount of THC, the active ingredient in ordinary cannabis, is considered to be up to 7.5 grammes.
The state court must now decide on a new punishment for the man concerned after the Supreme Court overturned their original sentence.
"Legal highs" is a term used to describe drugs sold as herbal mixtures, bath salts, air fresheners or plant food, which experts judge to be dangerous because of their unknown effects.