Seizures, its worst side effect, have been reported but mostly in the early days of its use (from the mid-1980s to mid-1990s) before safety limits were first defined in 1998. Nowadays, seizures are more likely when TMS is used in patients on drugs that may lower the brain's electric threshold for fits. These include medications ranging from antihistamines to antimalarials like chloroquine, antibiotics like ampicillin and cephalosporins, as well as recreational drugs such as Ecstasy, gamma-hydroxybutyrate (the date rape drug), and alcohol.

More common than seizures are fainting spells. But these generally last under a minute. The commonest side effect is sheer unpleasantness.

TMS can cause muscles in the scalp or upper face to twitch, resulting in a specific and unwelcome skin sensation. It can also cause headaches, toothaches or neckaches.

These and other adverse side effects aside, repetitive use of TMS over a long duration can lead to persistent changes in the way the human brain functions. This is probably because TMS actually induces lasting changes to the way that synapses - those connections between neurons in the brain - function, rendering them more robust and more efficient.

These long-lasting modifications in synaptic strength and efficiency point to what is called synaptic plasticity. This mechanism has long been identified as that which enables thought, learning, memory and behaviour. This may well be the organic basis for why TMS seems to help in some psychiatric disorders.

Particularly well attested to in recent studies is its efficacy in reducing auditory hallucinations ('hearing voices') in schizophrenia. But there is no systematic research on the cumulative psychiatric side effects of its long-term use.

Moreover, TMS has been shown to alter blood flow and oxygen levels in the human brain. A few studies suggest that it might also have a cumulative effect on one's immune defence mechanisms.

All things considered, therefore, at the present time, you should agree to be treated with TMS or participate in research in which it is employed only with the utmost circumspection.

andyho@sph.com.sg

This article was first published in The Straits Times.