There are a number of risks and complications of laser including:
Burning/blistering/scarring – as lasers heat up your thread veins, the skin can also overheat and occasionally blister, and even potentially scar. This scarring is a relatively rare complication though.
Bruising – newer methods tend to aim to reduce the chances of bruising when treating thread veins, but occasionally bruising may be unavoidable. The bruising usually fades over 10-14 days.
Partial/non response – as described above, not all thread veins will respond to treatment with a laser. After a number of treatments, any remaining veins are probably resistant to that particular laser, and another type of laser or alternative treatment may be considered.
Recurrence – often, the treated thread veins will never recur, but occasionally they can. This is likely due to blood flow being diverted from the treated veins into adjacent ones, causing them to swell and become visible. This may need further laser treatment.
Change in pigmentation – occasionally, at the same time as the laser treats the thread veins, the colour of the skin itself may also be affected in the treated area. This can result in hypopigmentation (paleness of skin) or hyperpigmentation (darkening of skin). This effect may be temporary and last a few months, or can occasionally be lifelong. The risks are higher with darker skin types.
Pain – the treated area can feel slightly sore or warm after treatment, which usually responds well to simple painkillers. Any pain is usually short-lived and lasts less than a day or two.
Infection – blistered and broken skin can allow infection to enter, resulting in a red inflamed area. This may require antibiotics, but could potentially increase the chances of scarring and pigment changes. Cold sores can be activated if the lip area is treated, and a laser specialist may prescribe antibiotics around the time of laser treatment to reduce this risk.