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Context. While some galactic bars show recent massive star formation (SF) along them, some others do not. Whether bars with low level of SF are a consequence of low star formation efficiency, low gas inflow rate, or dynamical effects remains a matter of debate. Aims. In order to study the physical conditions that enable or prevent SF, we perform a multi-wavelength analysis of 12 strongly barred galaxies with total stellar masses log10(M⊙/M⊙)? [10.2, 11], chosen to host different degrees of SF along the bar major axis without any prior condition on gas content. We observe the CO(1-0) and CO(2-1) emission within bars with the IRAM-30 m telescope (beam sizes of 1.7-3.9 kpc and 0.9-2.0 kpc, respectively; 7-8 pointings per galaxy on average). Methods. We estimated molecular gas masses (Mmol) from the CO(1-0) and CO(2-1) emissions. SF rates (SFRs) were calculated from GALEX near-ultraviolet (UV) and WISE 12 μm images within the beam-pointings, covering the full bar extent (SFRs were also derived from far-UV and 22 μm). Results. We detect molecular gas along the bars of all probed galaxies. Molecular gas and SFR surface densities span the ranges log10(ςmol/[M⊙ pc-2]) ? [0.4,2.4] and log10(ςSFR/[M⊙ pc-1 kpc-2]]) ? [-3.25, -0.75], respectively. The star formation efficiency (SFE; i.e., SFR/Mmol) in bars varies between galaxies by up to an order of magnitude (SFE ? [0.1, 1.8] Gyr-1). On average, SFEs are roughly constant along bars. SFEs are not significantly different from the mean value in spiral galaxies reported in the literature (∼0.43 Gyr-1), regardless of whether we estimate Mmol from CO(1-0) or CO(2-1). Interestingly, the higher the total stellar mass of the host galaxy, the lower the SFE within their bars. In particular, the two galaxies in our sample with the lowest SFE and ςSFR (NGC 4548 and NGC 5850, SFE ? 0.25 Gyr-1, ςSFR ? 10-2.25 M⊙ yr-1 kpc-2, M⊙ ? 1010.7 M⊙) are also those hosting massive bulges and signs of past interactions with nearby companions. Conclusions. We present a statistical analysis of the SFE in bars for a sample of 12 galaxies. The SFE in strong bars is not systematically inhibited (either in the central, middle, or end parts of the bar). Both environmental and internal quenching are likely responsible for the lowest SFEs reported in this work. © ESO 2021. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska Curie grant agreement No 893673. We acknowledge financial support from the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No 721463 to the SUNDIAL ITN network, from the State Research Agency (AEI-MCINN) of the Spanish Ministry of Science and Innovation under the grant "The structure and evolution of galaxies and their central regions" with reference PID2019-105602GB-I00/10.13039/501100011033, and from IAC project P/300724, financed by the Ministry of Science and Innovation, through the State Budget and by the Canary Islands Department of Economy, Knowledge and Employment, through the Regional Budget of the Autonomous Community. SDG acknowledges support from the Spanish Public Employment Service (SEPE). Furthermore, we acknowledge support by the research project AYA2017-84897-P from the Spanish Ministerio de Economia y Competitividad, from the European Regional Development Funds (FEDER) and the Junta de Andalucia (Spain) grants FQM108. DE acknowledges support from a Beatriz Galindo senior fellowship (BG20/00224) from the Ministry of Science and Innovation. LVM acknowledges financial support from the grants AYA2015-65973-C3-1-R and RTI2018-096228-B-C31 (MINECO/FEDER, UE), as well as from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This research makes use of python (http://www.python.org), Matplotlib (Hunter 2007), and Astropy (Astropy Collaboration 2013, 2018). We acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr). ; Peer reviewed