The always interesting site Sare Antifaxista[es/eu] mentions today that the Spanish Ministry of Interior (police and such) has spent in these times of crisis, austerity and cuts nothing less than €1 billion in tear gas weapons and ammo.
As counter measure and with the disclaimer of the information being offered only for self-defense purposes, they offer us today a poster with instruction in Spanish on how to counter the way-too-fashionable capsacin-based weapons:
The self-defense method consists in a painter mask and protection glasses (both normally available in hardware shops). These should act as preemptive protection if worn when the tear gas is thrown.
But in case that you or other people are anyhow affected, they propose the following method: get a spray bottle and clean it very well, dilute anti-acid medicine in liquid form (the trade-marks may vary from country to country) with water half-and-half. If affected by the tear gas or equivalent, spray the anti-acid solution on eyes and mouth and swallow.
Apparently there is even a study by the University of San Francisco that confirms that the use of anti-acid medicine against capsacine-based gases is effective, specially if the counter-measure is performed soon after the attack.
They also recommend to keep all protests peaceful in order to be taken seriously and get your message heard.