Short answer: in most cases it is harmless and costs you almost nothing in electricity, but there is one version of it that you should fix, because it means the fitting is live even when the switch is off. Here is how to tell which one you have.
There are four mechanisms that produce a dim glow from an LED lamp that is supposed to be off, and they have completely different implications.
The first is switched neutral. In a lot of older installations the switch breaks the neutral conductor instead of the live one. The lamp is then connected to live permanently, and it only stops glowing because the return path is interrupted. With an incandescent bulb you would never notice. With an LED you often get a faint glow, because the live conductor running to the fitting couples capacitively to anything nearby, and that is enough to push a fraction of a milliamp through the driver. If this is your situation, the fitting is live at all times, which matters the moment someone changes a bulb or touches the terminals. This is the one worth getting an electrician to correct, and it is a ten minute job if the cables allow it.
The second, and by far the most common in houses that have been upgraded, is standby leakage through a smart switch. A switch that has no neutral at the box has to power itself somehow, and the only place it can take energy from is the tiny current that flows through the lamp. With an old filament bulb that current did nothing you could see. An LED needs so little energy that the same leakage is enough to make it glow or, worse, blink every few seconds. This is not dangerous and it does not damage the lamp, but it is genuinely annoying, and it is the single most common complaint about no-neutral smart switches.
The third is capacitive coupling between cables. If the cable feeding the light runs alongside another live circuit for several metres in the same conduit or trunking, you get the same effect without any smart switch involved at all: a few tens of volts of induced potential, no real current behind it, but enough to light an LED faintly. Long runs to a garage or an outbuilding are the classic case.
The fourth is a switch with a built-in indicator lamp or a neon locator. Those deliberately pass a small current through the load to light the indicator, which again was invisible with old bulbs and very visible with LEDs.
Working out which one you have is quick. Turn the light off and look at it for a full minute in a dark room. If it is a steady faint glow, suspect switched neutral, coupling, or an indicator. If it pulses or blinks periodically, suspect a smart switch's standby circuit charging and discharging. Then, if you are comfortable doing so safely, switch off at the consumer unit, remove the switch plate, and disconnect the switched conductor. If the lamp still glows with the switch entirely out of circuit, the cause is coupling or switched neutral rather than the switch itself. Anything involving measuring voltage at the fitting should be done with the breaker off and confirmed dead first, and if any of that is unclear, this is exactly the point where an electrician is cheaper than a mistake.
The fixes, in the order I would try them:
If the switch is breaking neutral, have it rewired so it breaks live. No product solves this properly, because the problem is which conductor is being interrupted, not what is doing the interrupting.
If it is a no-neutral smart switch, the clean fix is to move to a switch that has a real neutral available, which powers itself properly and stops leaking through the lamp. If there is no neutral in the box, the next best thing is a capacitor module fitted across the lamp terminals, which gives the leakage current somewhere to go other than through the LED. It needs to be a properly rated X2 class part, fitted by someone who knows what they are doing. The third option is to buy a lamp whose driver has a bleeder circuit, which simply stops it responding to such small currents. Some do, and it is not written on the packaging, so you end up trying one.
If it is coupling on a long run, the capacitor module works there too, and so does reducing how far the lighting cable runs parallel to a live circuit.
What I would not do: fit a resistor across the lamp to soak up the current, because it dissipates heat in a ceiling void or a fitting for no good reason, and do not ignore it on the assumption that it must be fine.
On the safety question specifically: a glow caused by leakage or coupling is real but tiny, typically a fraction of a milliamp at a few tens of volts. It is not a fire risk, it does not meaningfully shorten the lamp's life, and the energy involved is far below what your meter can register. The reason not to ignore it is not the glow itself, it is that one of the possible causes means the fitting is live when you think it is not.
I work on the manufacturing side of smart switches, so I see this from the other end: it is the most returned-with-a-complaint behaviour of any no-neutral design, and the honest answer is that it is inherent to powering a device from the load. The designs that handle it best either require a neutral or specify a leakage current low enough that most lamps simply do not react. If a seller cannot tell you what their switch's standby leakage is, that is usually a sign nobody measured it.
If it helps to see what is on the market, our switch range is here: https://xingliaoshop.com/product-category/smart-switch/
Xingliao Smart Home builds smart switches, video doorbells and security cameras on Tuya WiFi and Tuya ZigBee, and runs OEM/ODM programmes from Shenzhen. Factory and ODM capability.
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