Viagra (sildenafil) is a prescription medicine, so ideally your husband should be assessed by a doctor and it prescribed by them.. we are on BL and a lot of people take prescription meds that haven't been prescribed to them here.
There are a few conditions that could be dangerous with viagra - low blood pressure, severe liver or kidney disease, recent heart attack/stroke, use of a drug in the nitrate class (like GTN) or people for whom sexual intercourse poses a risk to (usually people with severe heart disease). The main side effects are headache, flushing, blurred vision, nasal congestion and priapism (an erection that doesn't go away and needs urgent medical attention). Rarely it can also cause visual problems, and sex itself taxes the heart and circulation - it is fine for most people, but as I mentioned if your husband has heart disease then it can put it under extra strain and can precipitate heart attacks (this is rare). If your husband is concerned about any of these things, he should stop taking it and see a doctor..
Yes, it is supposed to be for people with erectile dysfunction, but a lot of people take it to enhance their sex life. It sounds like both you and your husband have a pretty high sex-drive - if you are happy with how things are then that is fine, but it might be better to have a discussion with your husband about your sex life and work out something that suits you both, to avoid the need for him to take a medication like viagra off prescription.. several times a day is higher than the average amount of sex couples get, but if it suits you both then it is absolutely fine! However taking a medication so you can have sex 4-5 times a day rather than 2-3 seems a little extreme to me.. there are other things your husband can do to please you apart from full intercourse, maybe that would be a better plan? Your husband might be feeling insecure too, and needing a bit of reassurance that you are happy with him and your sex life the way it is without hik needing to take viagra..
Good luck! Bear in mind this is not a medical site and is no substitute for seeking medical attention. If you like I can move this to Sex, Love and Relationships if you want to discuss your current situation further..