Hello, Phentermine is a weight loss and like many weight loss drugs,it has few side effects and sexual side effects are one of them. Many people complain of loss of sexual desires and erectile dysfunction after using this drug.But generally such effects diminish on continued use of the drug. Many other people have even complained of diminishing of size of testicles and penis ,however all such side effects are temporary and tend to go away with continued use of drug. It is generally advised to continue the drug and see the side effects. Along with use of the drug,person should indulge in healthy eating and regular exercises. All these actions of the person results in weight loss over a period of couple of months which in itself would boost and increase your sex drive and coupled with waning of side effects of drug,a person shall regain and rather increase his sexual drive. If after a period of couple of few months you feel that your sex life is still in mess,you may ask your doctor for change of medication.  In nutshell you must wait for sometime before dismissing the drug. Thanks.