It’s unclear how long any effect may last, but the label recommends no more than one injection in 24 hours and no more than eight injections per month. It increases blood flow to the penis to result in an erection. Often, these are prescribed for chest pain called angina. You can take it up to 4 hours before you want to have sex. For example: Many women find that the stresses of daily life reduce their desire for sex.In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 hours to start. The Food and Drug Administration (FDA) has not approved Viagra (sildenafil) for use in women. However, your healthcare provider may prescribe it for off-label use in female sexual arousal disorder (FSAD)–a type of sexual dysfunction characterized by low sex drive (libido) in females.  · The FDA hasn’t approved Viagra (sildenafil) itself for people with female anatomy to use. However, it has been prescribed off-label for AFAB individuals with a low libido. OFF-LABEL DRUG USE.