The invention relates to a thermoformed pharmaceutical dosage form having a breaking strength of at least 300 N, said dosage form comprising - a pharmacologically active ingredient (A), - a free physiologically acceptable acid (B) in an amount of from 0.001 wt.-% to 5.0 wt.-%, based on the total weight of the pharmaceutical dosage form, and - a polyalkylene oxide (C) having a weight average molecular weight M w of at least 200,000 g/mol.本発明は、少なくとも300Nの破壊強度を有する熱成形医薬剤形に関し、前記剤形は、オピオイド(A)と、医薬剤形の総重量に基づいて0.001~5.0重量%の量の生理学的に許容可能な遊離酸(B)と、少なくとも200,000g/molの重量平均分子量Mwを有するポリアルキレンオキシド(C)とを含む。