Hi,
Prostate are the male genital organs which are not responsible for any libido like activity. This is the function of the hormone testosterone which is secreted by the testes. Kindly do take him off viagra as early as possible and do put your husband through counseling sessions.
Mood swings due to hormonal changes are popular opinions, however, the source of male hormones are the testes not the prostate. It is more likely that your husband has problems with erectile function and becomes frustrated. One of 5 patients after prostatectomy may need to take Viagra initially, the most successful in weaning themselves from it are those who are able to maintain regular erections. Viagra generally affects blood vessels and is not a hormonal drug, hence mood changes are not associated with it. Hence, the need to take Viagra for now may indirectly contribute to his frustration with an altered sense of self. Perhaps, counseling would help, or even reassurance that erectile dysfunction is a common problem and openness is the first step to finding answers.