Don’t trouble yourself with this. It’s only a page long and the messages for employers are shorter still:-
(i) many “legal highs” are actually illegal, but the distinction is broadly irrelevant for workplace purposes. You should include them as barred under your drugs and alcohol policy, but focus on their impact on the employee’s behaviours and ability to work, not whether they are legal. Acas recommends training managers to spot the signs of substance abuse, legal or not. However, since legal highs can act variously as both “uppers” and “downers” this may with respect be easier said than done.
(ii) it is hard to test for legal highs in the same way as you might for “normal” drugs, because no one really knows what they contain.
(iii) if you treat addictions to drugs or alcohol as an illness in the first instance rather than a disciplinary matter (i.e. you encourage/offer help or education or treatment) then you should do the same with legal highs. Conversely, if you take a harder line with drugs and alcohol (especially in safety-critical roles) then you can do so with legal highs also.
(iv) if you monitor employees’ internet usage, keep an eye open for repeated visits to sites flogging bath salts, incense or plant food. Apparently it is under such covers that legal highs are often marketed.
(v) but remain awake to the possibility that your employee is in his spare time merely an exceptionally finely-scented gardener – I still recall the embarrassment of a client many years ago which nearly dismissed a poorly-performing employee after the discovery of a small inlaid box of white powder hidden in his desk drawer. Those were the days when drugs in the City were far more prevalent than now, so it was to my client’s palpable disappointment that analysis showed the power actually to be a treatment for athlete’s foot.