I have taken Phentermine in the past and it does seem to lose it's effectiveness after awhile...I lost a ton and then gained it all back plus. I took it in the fen/phen combo and now have moderate mitral valve regurgitation....the phentermine was not the cause but my cardio specialist told me to completely stay away from it...it can cause heart palpatations, arrthymias and can be addictive....it works much like meth....on a chemical level. I have never used meth but I can tell you that the combo made me feel great and I lost alot of weight but the heart damage is certainly not worth the risk....Leslie
Taking my life back one pound at a time!!