Injection assistance device (1; 101; 201) comprising an injection device for injecting a product into an injection site, this injection device comprising at least one hollow body intended to receive a product that is to be injected, at least one hollow injection needle intended to penetrate the injection site, and at least one piston plunger housed in the said body, the said body and the said piston plunger being able to be moved in axial translation one with respect to the other, characterized in that the said injection assistance device (1; 101; 201) comprises at least: - grasping means (4; 75; 106; 206; 228) intended to be manually handled by the user in order to apply said injection device on said injection site during an insertion and an injection steps, said grasping means receiving at least in part the said body and being arranged in such a way as to allow the said body axial mobility between at least a first position known as the initial position in which the said needle is not exposed over its insertion length, and a second position known as the insertion position in which the said needle is exposed by a predetermined insertion length L, - at least first elastic return means (9; 57; 66; 76; 83; 111; 139; 209) coupled to said body and to said grasping means (4; 75; 106; 206; 228), said first elastic return means (9; 57; 66; 76; 83; 111; 139; 209) being in a partially expanded state when said body is in the insertion position, so as to dampen limited movement of said grasping means (4; 75; 106; 206; 228), in at least one of the two directions, respectively distal or proximal, during said injection step, said injection device being arranged in such a way that, in the insertion position, said grasping means (4; 75; 106; 206; 228) are separated from said body, or from an element (8; 104; 136) coupled to said body at least in said insertion position, by a gap (301; 401; 501; 601; 701; 801; 901; 1001; 2001) that allows the said grasping means (4; 75; 106; 206; 228) to move with respect to said body or to said element (8; 104; 136), to maintain said body in the insertion position and said needle at the predetermined insertion length L during the injection step, when the user increases a distal pressure on the grasping means (4; 75; 106; 206; 228) or when the user releases a distal pressure on the grasping means (4; 75; 106; 206; 228). - grasping means (4; 75; 106; 206; 228) intended to be manually handled by the user in order to apply said injection device on said injection site during an insertion and an injection steps, said grasping means receiving at least in part the said body and being arranged in such a way as to allow the said body axial mobility between at least a first position known as the initial position in which the said needle is not exposed over its insertion length, and a second position known as the insertion position in which the said needle is exposed by a predetermined insertion length L, - at least first elastic return means (9; 57; 66; 76; 83; 111; 139; 209) coupled to said body and to said grasping means (4; 75; 106; 206; 228), said first elastic return means (9; 57; 66; 76; 83; 111; 139; 209) being in a partially expanded state when said body is in the insertion position, so as to dampen limited movement of said grasping means (4; 75; 106; 206; 228), in at least one of the two directions, respectively distal or proximal, during said injection step, said injection device being arranged in such a way that, in the insertion position, said grasping means (4; 75; 106; 206; 228) are separated from said body, or from an element (8; 104; 136) coupled to said body at least in said insertion position, by a gap (301; 401; 501; 601; 701; 801; 901; 1001; 2001) that allows the said grasping means (4; 75; 106; 206; 228) to move with respect to said body or to said element (8; 104; 136), to maintain said body in the insertion position and said needle at the predetermined insertion length L during the injection step, when the user increases a distal pressure on the grasping means (4; 75; 106; 206; 228) or when the user releases a distal pressure on the grasping means (4; 75; 106; 206; 228).