In principle in Germany, the possession of even the smallest amount of illegal drugs is a criminal offence and will be prosecuted. In practice, however, German police and prosecutors use a pragmatic approach: for Cannabis (Marijuana) there is an unofficial threshold of (depending on where you were caught) anywhere between 5 grams (in very strict and conservative German states like Bavaria) and 10 grams (in more permissive states like Berlin). The weight specification refers to pure THC (tetrahydrocannabinol), i.e. the active ingredient. This usually equals 30 to 50 grams of actual cannabis plant harvest material, depending on the quality. For more information on this small drug amount policy of German law enforcement see here.
If what the authorities find on the person is below that amount, there is a chance of convincing the German prosecutor to discontinue the investigation on payment of a small fine or even – depending on the circumstances – without a fine. If the amount is larger or if the offender has a criminal drug record, then there is a much more serious problem. The same is true if the prosecutor charges you for dealing drugs, even if only very small amounts.
In any case, the general advice when arrested by German police or customs is not to make any statements to the police. Contact a lawyer, who will ask for a copy of the prosecutor’s file. Only then, i.e. once the defence lawyer knows what the authorities know, it may be recommended that you submit a written statement. More practical advice in case of criminal prosecution in Germany here.
If you are accused of any crime in Germany, you should always demand to speak to an English speaking German lawyer. In the south of Germany you may contact attorney Bernhard Schmeilzl of the law firm Graf & Partners (++49 175 480 2209). For other parts of Germany we can recommend qualified criminal defence attorneys.
Basic information on law enforcement and criminal prosecution in Germany: The German Criminal Code (in English language) and the Code of Criminal Procedure are available here. German judge Joachimski here compares the practical aspects of US and German criminal procedure and here is a very informative essay on the German criminal procedure produced by the NZ Law Commision (download here: GERMAN CRIMINAL PROCEDURE – german_criminal_procedure).