I've taken 5mg finasteride everyday for a week now. Are there downsides to taking a large dose like this?

The FDA approved dose for treatment of hair loss in males with androgenetic alopecia (male balding) is 1 mg. 5 mg is not better than 1 mg and use of higher doses does increase the risk of side effects.

I typically do not recommend taking 5 g of Finasteride per day, since this will increase the potential side effect profile - sexual dysfunction, muscle pain, depression, anxiety, etc.  I typically will place people on 1 mg per day 5 to 7 days per week, but prefer that they take the weekend off and just take it during the week. 
Finasteride blocks the conversion of Testosterone to DHT, effectively blocking the main culprit for male patterned hair loss - DHT.  Unfortunately, DHT is also a parent hormone for a number of neurohormones in the brain and this is where the side effects can present themselves, with a host of symptoms called Post Finasteride Syndrome (PFS).  The amount of DHT blocked at the hair follicle with 1 mg is not much less than with 2.5 mg or even 5 mg, but the side effect profile and chance of PFS is reduced.

You can purchase generic finasteride at the 1mg dose very inexpensively today and if your doctor wants you to take two 1mg genetic finasteride, you can do this