The toxic chemicals can cause such as irritation, swelling, rashes and burns
Police are warning people about fake beauty products after substances such as rat droppings, human urine and arsenic were found in seized goods.
Make-up, perfume and sun cream are among the phony items being highlighted by the City of London Police.
It said lab tests showed counterfeit products also had toxic chemicals like arsenic, mercury and cyanide.
The campaign also warns about fake electrical beauty goods that could cause electrocution.
Counterfeit make-up is often produced in unhygienic factories and can contain rat droppings
The force said in the UK it is estimated at least £90m is spent every year on fake goods.
Counterfeit beauty products in particular are becoming increasingly common and easily available on the internet.
Police said laboratory tests have shown fake perfume often contains poisonous chemicals including cyanide and even human urine.
Phony cosmetics such as eyeliner, mascara, lip gloss and foundation have been found to contain toxic levels of chemicals and harmful substances such as arsenic, mercury and lead.
City of London Police have seized £3.5m worth of fake products
All of these can cause allergic reactions, such as skin irritation, swelling, rashes and burns as well as leaving the person with longer term health problems.
A City of London spokesman said counterfeit make-up is often produced in unhygienic factories and there have been cases where rats' droppings and poison have also been found in them.
Det Supt Maria Woodall, who oversees the Police Intellectual Property Crime Unit at City of London Police, said it had suspended more than 5,500 websites selling fake luxury branded goods as well as seizing more than £3.5m worth of phony products.
She also said customers' payment and personal details had been stolen to make other purchases.
"Beauty products are meant to enhance your features. However, the fakes can in fact do quite the opposite," she added.
"Our general rule is - if it seems too good to be true then it probably is."