Hi amit, Sexual performance depends on many things and one of the important thing is that if you are in stress your performance will decrease significantly. So stress and performance under pressure always make people failed in this field. Apart from stress many other factors are also there which may affect the performance. For best performance the person should be physically and mentally fit. Now as far as the size of your penis is concerned, there are no medication available to increase the size anatomical. Penis size varies according to the physical structure of the body and perform well in sex its size is not the only thing. For you information I would like to give you information that the outer 2 inch portion of the vagina has the most sensitive part and so if some one has the penis size which can penetrate the vagina, it is good for the sex. Another important point is the premature ejaculation and again the most important cause for it is stress and anxiety to perform. Some time it can be because of other reasons also, but stress and anxiety is most important one. The drug penegra has Sildenafil as its pharmacological agent. It is used for erectile dysfunction, that is when there is some problem with the penile erection. It is not going to increase of penis size or help you in premature ejaculation. The adverse effects which you mentioned are not associated with the use of the penegra. Diarrhea and typhoid are infectious types of disease and is not related to drug. Greying of hair because of sildenafil is not reported as adverse effect. These symptom shows that there is some problem with your general health and I would recommend you to see a doctor for the general check up. Also you should consult doctor for your thyroid problem. Avoid taking penegra if you don not have nay problem with erection as it is not going to help in increasing the penile size and premature ejaculation. Thanks and hope this will help you, take care.