I have been taking tamsulosin for 18 months ( I. I have been taking tamsulosin for 18 months ( I am 67 )and noticed some weight gain despite strict dieting and cutting down in meal sizes. How to use Tamsulosin HCL. Read the Patient Information Leaflet if available from your pharmacist before you start taking this medication and each time you get a refill. Weight gain is not documented as a side effect of Flomax (Tamsulosin). Among the clinical trials that were conducted for an in-depth research on the drug, the listed side-effects did not include weight gain. His wife asked his psychiatrist to find a substitute for the Depakote due to significant weight gain. The doctor prescribed Tegretol instead. Then my dad quit taking his Effexor (I'm not sure why) and his depression symptoms suddenly returned. Weight gain has not been reported as a side effect with Tamsulosin. Please consult your doctor in case you experience changes in weight while taking Tamsulosin. Can Flomax cause Weight Gain? Complete analysis from patient reviews and trusted online health resources, including first-hand experiences. Complete analysis from patient reviews and trusted online health resources, including first-hand experiences. Summary. Weight increased is found among people who take Tamsulosin, especially for people who are male, 60+ old , have been taking the drug for 1 month, also take medication Lyrica, and have High blood pressure. Common Tamsulosin Side Effects Tamsulosin has been thoroughly studied in clinical trials, with thousands of people having been evaluated. In these studies, side effects occurring in the group of people taking the drug are always documented and compared to the side effects that occur in a similar group of people not taking the medicine. An Overview of Flomax and Weight Gain There are a number of possible side effects with Flomax ® (tamsulosin hydrochloride). However, weight gain does not appear to be one of them. Side Effects. Drug information provided by: IBM Micromedex. Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.