Under-reporting of ADR may be associated with poor knowledge, attitudes and practices to pharmacovigilance. This study evaluated knowledge, attitudes and practices of healthcare professionals about ADR monitoring and reporting following interventions. This longitudinal study included 36 healthcare professionals participating in ART program in a tertiary hospital. Interventions included group training on pharmacovigilance (PV) and provision of ADR reporting forms amongst others. Assessments were conducted at months 0 and 6 post-interventions using study-specific Likerttype instruments. Mean attitude scores above midpoint of 3.6 on 5-point scale were regarded as positive and below as negative. P<0.05 used to determine statistical significance. Mean age of participants was 36.6 (95%CI, 34.5–38.7) years; 61.1% males; 44.4% doctors, 13.9% pharmacists, 19.4% nurses, 8.3% laboratory scientists, 8.3% record officers and 5.6% welfare officers. None had received training on PV previously. Mean knowledge test score increased from 53.6% (95%CI, 44.6–63.6) at pre-intervention to 77.1% (95%CI, 72.8–81.4) at post-intervention with a mean change of 146.9% (95%CI, 60.5–233.3; p=0.000). Mean rated attitude scores increased from 3.6 (95%CI, 3.4–3.8) at pre-intervention to 4.2 (95%CI, 4.0–4.4) at post-intervention; the difference was statistically significant (p=0.000). 75.8% reported that ADR reporting forms were not readily available at pre-intervention compared to 18.2% at postintervention; 15.2% had reported ADR previously at pre-intervention compared to 69.7% at post-intervention; 12.1% reported providing information regarding ADRs and its management always at pre-intervention compared to 45.5% at post-intervention; these differences were statistically significant (p<0.05). Lack/inadequate knowledge, unavailability of reporting forms and negative attitudes were barriers identified; and addressing them resulted in significant improvement in this setting. Scaling up these interventions to other hospitals can better the situation of under-reporting of ADRs in Nigeria.