Methods for preventing or alleviating the loss of drilling fluids and other well treatment fluids into a subterranean formation during drilling or construction of boreholes therein include a drilling fluid including a lost circulation material. The lost circulation material includes a suspension of swelled particles of a pH sensitive polymeric material, the swelled particles capable of reversibly attaching to other swelled particles of the polymeric material. The pH of the aqueous solution is such that each particle of the swelled pH sensitive polymeric material is not attached to other swelled particles, and wherein upon lowering the pH of the suspension, the swelled particles attach to each other.