A long time ago the first human beings discovered the astonishing effects that catnip has on cats. Delighted by the amusing behavior of a cat being "high" on catnip they created toys filled with that herb. Those toys are nowadays pretty common and almost every cat has its own "dope" at home.
A small Documentary about Catnip and its Consequences
Catnip belongs like the common peppermint herb to the mint family. Its a native resident in southern Europe, Africa and Asia.
Neither the catnip that's filled into the toys nor the catnip that grows on our balcony or garden is directly noxious for cats. (But beware: There is one exception: The Matabi plant is related to common catnip and causes a strong addiction that makes cats stop eating and drinking until they die in some cases. The Matabi plant, though, only grows in certain parts of Asia and is not for sale in Europe or Northern America.)
The component that's so incredibly attractive to cats is the essential oil within the catnip plant. Cat's can smell and find it from kilometers away. Just like peppermint oil the pure oil of the catnip plant in high concentrations leads to skin irritations of both cats and human beings.
The concentrated oil, though, is usually nowhere for sale and so rare that it isn't dangerous to any cat and its owner at all.
There is another point, though, that's got a more dangerous character than the possible irritations catnip oil can cause (especially since almost every essential oil causes skin irritations used in a high dosage): It's the condition a cat is transferred to when confronted with catnip.
Catnip seems to be a Hallucinatory
Catnip causes trance-like conditions for approximately 50% of the cat population. Those conditions can be compared with trips human beings get on LSD.
A usual catnip-"trip" takes between 5 and 20 minutes - depending on the dosage of the herb or its oil, the age of the leaves (fresh catnip makes higher than dried leaves) and the kind of catnip.
The cat incessantly sniffs the catnip, licks the leaves, gnaws them, rolls over them and rubs its fur on the plant. The cat is in a condition far away from reality. It knows nothing but the plant as long as the trip takes.
Some cats even seem to have visions impassively staring into the void, haunting imaginary prey or cutting capers.
Catnip in Moderation is Harmless
You as a cat owner should overthink your attitude toward catnip. It's more than a mere toy. It's a druglike device that can lead to negative consequences like the loss of reality. Or would you hand out marihuana or LSD to people you like for your own amusement if it was legal? Think about it!
On the other hand cats don't seem to get abstinence syndromes (like a headache) after a catnip-"trip". Therefore the occasional play with a catnip toy is neither dangerous nor reprehensible.
Allowing your cat to have a "trip" once a week, can't really be harmful. Most of us can openers also drink a glass of vine or beer on our weekend!