How is Viagra Prescribed? Viagra comes in tablet form in dosages of 25mg, 50mg and 100mg. * Men Up to 65 Years of Age - Viagra 50mg is prescribed to be taken once a day, 30 minutes to 4 hours before intercourse. In situations where 50mg isn't effective, your doctor may prescribe 100mg to be taken once daily 30 minutes to 4 hours before intercourse. * For Men Older than 65 Years of Age - Viagra 25mg is prescribed to be taken 30 minutes to 4 hours before intercourse. In some circumstances, men older then 65 may have their doses increased. Important Caution - Under no circumstances should men increase their Viagra doses without first consulting their physician.