Thermoformed pharmaceutical dosage form having a breaking strength of at least 300 N, comprising an opioid (A), a free physiologically acceptable acid (B) in an amount of from 0.001 to 5.0 wt.-%, based on the total weight of the pharmaceutical dosage form, anda polyalkylene oxide (C) having a weight average molecular weight Mw of at least 200, 000 g/mol.