One of the subsitute narcotics on sale in a Dublin head shop last year

Imagine going into a supermarket and picking up a mandarin, and when you're just about to put it in your basket, a staff member runs over screaming "you can't have that!" and slaps it out of your hand, before offering, "hey, try this clementine, it's just as good."

It's a weird situation – you're not allowed have exactly what you want, but you can get something very similar, easily, and even with a smile.

This pretty much sums up the current situation with drugs in Ireland, Britain and beyond. In the burgeoning world of recreational drugs (let's leave the slightly more ruthless cocaine, and the incredibly destructive heroin aside for a moment), legal highs are rapidly catching up with their illegal counterparts. Cheap, legal, convenient and effective, legal highs ? namely imitations of MDMA, cannabis and speed ? are becoming a remarkably successful consumer product. But does their very existence serve mostly to mock the ineffectiveness of our drug laws?

In so-called headshops around the country, teenage boys and girls out for the night, businessmen, homeless people and tourists queue for herbal ecstasy, 'snow' (a speed imitation) and other drugs they pay for legally and consume legally. Online, business is booming, with perhaps the greatest success story of legal highs ? mephedrone ? flying off the virtual shelves. Sold and marked as 'plant food' for about €30 a gram, mephedrone, meph, or meow, has become hugely successful simply because it works. Combining the effects of speed and ecstasy at a cheap and legal price, it seems to be more common than cocaine at clubs and parties these days, a remarkable achievement for a drug that only became available in 2008.

We don't know too much about mephedrone other than it seems to be a little more iffy than the drugs it aspires to be. When snorted, users have experienced acute burning in their nostrils, unlike the more forgiving cocaine or speed. The so-called come-down a day or few hours later is meant to be horribly heavier than one associated with MDMA, the main ingredient of ecstasy. A recent survey of mephedrone users by the National Addiction Centre in the UK revealed that over half of all users suffered unusual headaches and almost half experienced heart palpitations. That doesn't sound good.

The same goes for herbal pills, capsules and chunky tablets that aim to recreate the feeling of taking ecstasy. Users have experienced a far harder buzz, vomiting and horrific hangovers. It reminds me of the message that rings out over Glastonbury's infamous Stone Circle, effectively a drug supermarket during the day, where dealers walk around like old hands on Moore Street shouting "coke, ketamine, pills, skunk, acid", but others yell more vocally, "don't buy legal highs, they will make you sick".

The fact that drugs which probably should be illegal are being sold legally in Ireland just because they're a variant of a chemical equation throws up lots of interesting questions. Headshops aren't regulated, of course, so even though Irish businesses make millions in annual profits, they occupy a sort of "don't ask, don't tell" policy on our urban landscape. It's a bit like the project of Hamsterdam in The Wire, where drug dealing is pushed into a specific geographical area, tolerated and monitored, and no one gets hurt. Kind of. But, unfortunately, people do get hurt. Anyone can walk in and buy legal highs; any kid who passes for 18, anyone suicidal, anyone with a psychiatric illness, anyone who really should know better. And why shouldn't they? It's legal. Legalisation has always been contentious, not because of real research on how it would impact on society but because it goes against the widely held War on Drugs belief that drugs undermine society, and rolling back on that belief would signal some kind of moral defeat.

Unfortunately, many of our drug laws are based on astounding ignorance. The same ignorance that sees the relatively harmless ecstasy graded as a Class A substance. The same ignorance that sees our politicians hopelessly and embarrassingly uninformed about drugs and drug policy. The same ignorance that saw gardaí recently roll back on what they thought an ecstasy pill was worth and revise street values. They should have just asked their kids.

So, the government has to ask itself: should drugs that cause certain effects be illegal or not? Either you ban all drugs in one group, or legalise them all. But being so far behind to begin with, they're already fighting a losing battle. Every time a previously legal high gets banned in a jurisdiction, the factories in Britain and China and elsewhere just tweak the chemical equation, change the name and sell it again. Surely it's time for the legislature to address this new reality.