A men's shampoo that boasts it is "German Engineering for your hair" could not prove it reduced hair loss.
Newspaper adverts for Alpecin Caffeine C1 Shampoo said "Shampoo is too small a word for it.
"Alpecin provides caffeine to your hair, so it can actually help to reduce hair loss."
It then added users should leave the shampoo on for two minutes daily before washing off "to help the Caffeine Complex penetrate your hair and scalp."
But the advertising watchdog received a complaint from a consultant trichologist - an expert in hair and scalp - who questioned the hair loss prevention claim.
The Advertising Standards Authority banned the misleading ad ruling the firm did not have evidence to prove the shampoo could help reduce hair loss as advertised.
The manufacturer Dr Kurt Wolff GmbH & Co KG based in Bielefeld provided eight full studies, several study summaries and a consumer opinion survey which they believed supported the claim.
It added it had followed advertising and Medicines and Healthcare products Regulatory Agency guidelines to ensure the claims were cosmetic and did not imply any medicinal action.
It said Alpecin was a topically-applied caffeine that had a long history of use with individuals suffering from thinning hair or increased hair loss, generally referred to as androgenetic alopecia (AGA).
It added: "Under the influence of testosterone, hair follicle growth had been shown to decrease, resulting in hair being shed prematurely.
"Caffeine was able to counteract the suppression of hair growth induced by testosterone and even stimulate hair growth to the normal level."
But ASA said consumers would understand the claim to mean the shampoo "would result in a reduced rate and quantity of hair loss.
"We noted that there were a number of different causes of excess hair loss.
"In the absence of any qualification, we considered the claim implied that the product could reduce hair loss from any of those causes in both men and women.
"Because the claim referred to reducing, and not preventing or curing, hair loss, we considered that it would not be understood as medicinal.
"Nonetheless, Alpecin was required to hold adequate evidence to support the claim.
"We noted that the product had only been tested on subjects with androgenetic alopecia, which mainly affected men.
"Taking into account the body of evidence as a whole, we considered that we had not seen any studies of the actual product as used by consumers on their scalp using an accurate and objective analysis of hair growth, in a well-designed and well-conducted trial.
"We concluded that the claim 'it can actually help to reduce hair loss' had not been substantiated and was therefore misleading.
"The ad must not appear again in the form complained of.
"We told Dr Kurt Wolff GmbH & Co KG trading as Alpecin not to state or imply that their product could reduce hair loss unless they held adequate evidence to support their claims."