A critical element was plainly enabling the patient - in this case a male from an unknown ancient culture similar to the Karasuk people, but not one of them - to enter an 'altered state of mind' in which the pain was minimised.
Plants supposed as being used as painkillers in other locations are absent from Siberia, and instead Slepchenko and his colleagues identified flora which were used by indigenous peoples here to used to bring a person to an altered state of mind.
There are an intriguing number of such potential painkillers: for example, some ethnic groups used juniper and thyme in Shamanic practices; the Nivkhi people burned wild rosemary sticks and leaves; northern indigenous peoples used fly agaric mushrooms (Amanita muscarica), a powerful hallucinogen.
The 'most obvious', however, was probably cannabis.
But the consumption of fungi, together with other Shamanic practices, such as ecstatic dancing or the use of a drum, is seen as a likely method of altering the conscious state of a patient and so reducing pain to the extent necessary to carry out surgery.
This man was not by any means the earliest known case of trepanation in Siberia: there are, for example, some dating to the Neolithic period.