Why do cats react so strongly to catnip?

In a word, it is because they are junkies. The catnip plant, a member of the mint family, contains an oil called hepetalactone, an unsaturated lactone which does for some cats what marijuana does for some people.

When cats find this plant in a garden they take off on a ten-minute 'trip' during which they appear to enter a state of ecstasy. This is a somewhat anthropomorphic interpretation because we have no idea what is really happening inside the cat's brain, but anyone who has seen a strong catnip reaction will know just how trancelike and drugged the animal seems to become. All species of cats react in this way, even lions, but not every individual cat does so, There are some non-trippers and the difference is known to be genetic. With cats, you are either born a junkie or you are not. Conditioning has nothing to do with it.

Under-age cats, incidentally, never trip. For the first two months of life all kittens avoid catnip, and the positive reaction to it does not appear until they are three months old. Then they split into two groups those that no longer actively avoid catnip, but simply ignore it and treat it like any other plant in the garden, and those that go wild as soon as they contact it. The split is roughly 50/50, with slightly more in the positive group. The positive reaction takes the following form: the cat approaches the catnip plant and sniffs it; then, with growing frenzy, it starts to lick it, bite it, chew it, rub against it repeatedly with its cheek and its chin, head-shake, rub it with its body, purr loudly, growl, miaow, roll over and even leap in the air.

Washing and clawing are also sometimes observed. Even the most reserved of cats seems to be totally disinhibited by the catnip chemical. Because the rolling behaviour seen during the trancelike state is similar to the body actions of female cats in oestrus, it has been suggested that catnip is a kind of feline aphrodisiac. This is not particularly convincing, because the 50 per cent of cats that show the full reaction include both males and females, and both entire animals and those which have been castrated or spayed. So it does not seem to be a 'sex trip', but rather a drug trip which produces similar states of ecstasy to those experienced during the peak of sexual activity. Cat junkies are lucky.

Unlike so many human drugs, catnip does no lasting damage, and after the ten-minute experience is over the cat is back to normal with no illeffects. Catnip (Nepeta caturia) is not the only plant to produce these strange reactions in cats. Valerian (V~le~~na officinalis) is another one, and there are several more that have strong cat-appeal.

The strangest discovery, which seems to make no sense at all, is that if catnip or valerian are administered to cats internally they act as tranquillizers. How they can be 'uppers' externally and 'downers' internally remains a mystery.