Hi, Sunflower. If by seltzer water you mean tonic water, yes, it will can make you weaker. Why? It contains Quinine. That does the opposite of Mestinon. Other things to avoid, besides some drugs, are certain eye drops with Atropine.

Please always research what you are drinking or eating! Tonic water may not contain a lot of Quinine but, for some people, it's enough to make them much weaker.

Welcome to the forum, BTW.

Annie