An injection device comprising: - an injection needle assembly, comprising: - a needle cannula attached to a needle hub, the needle cannula being of elongated shape extending in a first direction along a needle axis from the needle hub towards a pointed tip at a free end (3a, 3b) of the needle cannula, and - a needle cover (10, 10a, 10b) forming an axially extending elongated flexible enclosure accommodating the needle cannula, wherein the needle cover (10, 10a, 10b) has a first end mounted relative to the needle hub and a second end extending beyond the pointed tip of the needle cannula, the needle cover being configured to axially collapse when a penetration force is applied on the second end of the needle cover towards the needle hub for penetrating the second end of the needle cover by the needle cannula, wherein the injection device further comprises a needle shield with a needle opening, the needle shield and the injection needle assembly being arranged axially slidably relative to each other for causing the needle cannula to penetrate the needle cover and protrude through the needle opening of the needle shield. characterized in that the needle cannula is selected as one of a 28 gauge needle cannula, a 29 gauge needle cannula and a 30 gauge needle cannula, and wherein the needle cover (10, 10a, 10b) defines: - a shaft section encircling the needle cannula and extending along said needle axis away from the first end, the shaft section defining annular collapse regions comprising wall areas of minimum wall thickness t 1, and - a bulb section having bulb wall areas (17a, 19, 17b) axially aligned with and encircling the pointed tip of the needle cannula and having a closed central end surface, the bulb section comprising wall areas (17a, 17b) having a wall thickness t 2 larger than said minimum wall thickness t 1, wherein the needle cover (10, 10a, 10b), in an initial non-penetrated state, assumes a first configuration wherein the wall areas of the bulb section and the wall areas of the shaft section are spaced apart from the needle cannula, and wherein the shaft section comprises one or more reinforcing ribs, each reinforcing rib extending non-parallel to the needle axis so as to at least partly encircle the needle cannula, wherein each reinforcing rib is disposed axially between the bulb section and the first end of the needle cover to divide adjoining annular collapse regions of the shaft section. - an injection needle assembly, comprising: - a needle cannula attached to a needle hub, the needle cannula being of elongated shape extending in a first direction along a needle axis from the needle hub towards a pointed tip at a free end (3a, 3b) of the needle cannula, and - a needle cover (10, 10a, 10b) forming an axially extending elongated flexible enclosure accommodating the needle cannula, wherein the needle cover (10, 10a, 10b) has a first end mounted relative to the needle hub and a second end extending beyond the pointed tip of the needle cannula, the needle cover being configured to axially collapse when a penetration force is applied on the second end of the needle cover towards the needle hub for penetrating the second end of the needle cover by the needle cannula, - a needle cannula attached to a needle hub, the needle cannula being of elongated shape extending in a first direction along a needle axis from the needle hub towards a pointed tip at a free end (3a, 3b) of the needle cannula, and - a needle cover (10, 10a, 10b) forming an axially extending elongated flexible enclosure accommodating the needle cannula, wherein the needle cover (10, 10a, 10b) has a first end mounted relative to the needle hub and a second end extending beyond the pointed tip of the needle cannula, the needle cover being configured to axially collapse when a penetration force is applied on the second end of the needle cover towards the needle hub for penetrating the second end of the needle cover by the needle cannula, - a shaft section encircling the needle cannula and extending along said needle axis away from the first end, the shaft section defining annular collapse regions comprising wall areas of minimum wall thickness t 1, and - a bulb section having bulb wall areas (17a, 19, 17b) axially aligned with and encircling the pointed tip of the needle cannula and having a closed central end surface, the bulb section comprising wall areas (17a, 17b) having a wall thickness t 2 larger than said minimum wall thickness t 1,