A pharmaceutical formulation, which is a solid formulation selected from the following embodiments, and: a pharmaceutical formulation comprising: (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) an acid that has a pKa of 4 or less, and (c) at least one further pharmaceutically acceptable excipient; a pharmaceutical formulation comprising: (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) optionally an acid that has a pKa of 4 or less, (c) at least one further pharmaceutically acceptable excipient and (d) wherein it is free of excipients capable of accepting proton, the excipients capable of accepting proton being selected from: - bases, - salts of weak acids, wherein the acid is having a pKa of more than 2, such as salts comprising an anion derived from carboxylic acid, silicic acid, phosphoric acid, and carbonic acid, and - salts of alcohols; and a pharmaceutical formulation comprising: (a) as the active ingredient, lenalidomide in the form of hydrochloride acid addition salt, (c) one or more pharmaceutically acceptable excipients, wherein (e) the excipients are selected such that the pharmaceutical composition shows acidic properties, which means that a suspension of the formulation in water exhibits a pH value in the range of 1 to 5, wherein the pH value is determined by measuring the pH value of a 20% w/v suspension prepared by dispersing a powdered form of the pharmaceutical formulation in an appropriate amount of purified water, using a pH meter. a pharmaceutical formulation comprising: (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) an acid that has a pKa of 4 or less, and (c) at least one further pharmaceutically acceptable excipient; (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) an acid that has a pKa of 4 or less, and (c) at least one further pharmaceutically acceptable excipient; a pharmaceutical formulation comprising: (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) optionally an acid that has a pKa of 4 or less, (c) at least one further pharmaceutically acceptable excipient and (d) wherein it is free of excipients capable of accepting proton, the excipients capable of accepting proton being selected from: - bases, - salts of weak acids, wherein the acid is having a pKa of more than 2, such as salts comprising an anion derived from carboxylic acid, silicic acid, phosphoric acid, and carbonic acid, and - salts of alcohols; and (a) lenalidomide in the form of a pharmaceutically acceptable acid addition salt, (b) optionally an acid that has a pKa of 4 or less, (c) at least one further pharmaceutically acceptable excipient and (d) wherein it is free of excipients capable of accepting proton, the excipients capable of accepting proton being selected from: - bases, - salts of weak acids, wherein the acid is having a pKa of more than 2, such as salts comprising an anion derived from carboxylic acid, silicic acid, phosphoric acid, and carbonic acid, and - salts of alcohols; and - bases, - salts of weak acids, wherein the acid is having a pKa of more than 2, such as salts comprising an anion derived from carboxylic acid, silicic acid, phosphoric acid, and carbonic acid, and - salts of alcohols; and a pharmaceutical formulation comprising: (a) as the active ingredient, lenalidomide in the form of hydrochloride acid addition salt, (c) one or more pharmaceutically acceptable excipients, wherein (e) the excipients are selected such that the pharmaceutical composition shows acidic properties, which means that a suspension of the formulation in water exhibits a pH value in the range of 1 to 5, wherein the pH value is determined by measuring the pH value of a 20% w/v suspension prepared by dispersing a powdered form of the pharmaceutical formulation in an appropriate amount of purified water, using a pH meter. (a) as the active ingredient, lenalidomide in the form of hydrochloride acid addition salt, (c) one or more pharmaceutically acceptable excipients, wherein (e) the excipients are selected such that the pharmaceutical composition shows acidic properties, which means that a suspension of the formulation in water exhibits a pH value in the range of 1 to 5, wherein the pH value is determined by measuring the pH value of a 20% w/v suspension prepared by dispersing a powdered form of the pharmaceutical formulation in an appropriate amount of purified water, using a pH meter.