Objective. The objective of the study was to determine the knowledge on effects of self-medication by the community in Nandi county Kenya. Design. The study was a descriptive cross-sectional study and Quantitative methods were adopted. Setting. The study was carried out in Nandi County. Nandi County was purposively sampled. The respondents were sampled by stratified random sampling among adult patients attending outpatient department at Mosoriot Sub-County Hospital, Nandi County, Kenya; the researcher divided the population into strata and drew a predetermined number using simple random sampling (n = 248) Analysis. Data was analyzed through descriptive statistics, chi-square test of independence and logistic regression. Main outcome measures. Knowledge on effects of self-medication Results. 79% (196) said they always use medicine without the doctor?s prescription. 63.3% (157) said that symptomatic diagnosis was a good way to confirm illness. Although 66.1% said they know the effects of using drugs without prescription, results revealed that 75% of the respondents that poor knowledge of the effects of self-medication. Chi square analysis showed that there was no statistically significant relationship between the knowledge on effect of self- medication and gender X2 (1, N=248) =0.22. Logistic regression was done and respondents who were single were 0.4 times more likely (OR=0.42, 95% C.I, 0.06-2.84) to belong to the ?poor knowledge ?group than the ?good knowledge? group compared to the widowed Conclusion. That sensitization of the community/ public on self-medication is important.