It would appear much like any other meds, these have slightly different effects on each individual. I have had the privledge (pain and delight depending) of using all 3 for the last several years. I have been given both samples and full blown (no pun) prescriptions for them, (friggen expensive as not totally covered by ins.) and have found many different dose usages, benefits, and of course sides associated with them.
Viagra 'should' be used on an empty stomach for the best results. With a fairly empty stomach it can start as soon as 20-25 mins, but as long as up to 40-45 mins if you have any food in you. Especially with the smaller 25mg dose. I find this amount the most useful, and allows me to finish within a reasonable time if needbe. Usually less than an hour if I rush.. (All of these meds in any dose delay my ejac and de-sensitize a bit, but again this varies with dose and individual.) At 50mg no way I can finish fast, and I only take this amount if I've worked 12 hours that day, worked out that night or played a sport, and am completely exhausted. Balls want the sex, body says no.. 50mgs will typically keep you up when your body is wasted tired. Takes forever to finish though.. Have lasted 2 hours minimum, and over 4 hours 'up and down' with water breaks.. Yikes.. At 100mgs, yes I get these and cut them, I have only tried a couple times on a full stomach and with a snoot full of booze. If you are stimulated, you could get wood if you were dead. Very difficult to finish. The headache, stuffy nose, flush red face, bluish or pinkish vision depends on the dose, and how long you last.. For me anyway. I drop 400-600 mgs Ibu with a snack, and a ton of water immediately after, and the sides are a shorter duration.
Levitra tends to work a bit faster for me, and even with food in my stomach. As fast as 15 mins with low food, and 20-25mins with a bit of food. Similar desensitivity and delayed ejac to Viagra, but not as stable for long durations. 10mgs seem right for a short romp of an hour give or take, but 20mgs will last a lot longer just like the Viag. The sides are a bit different for me with less stuffiness and a bigger headache, and the sides seem to last longer. I only use (in a combo, micro doses of 2 or 3 of them) or if I've eaten before sex.
Cialas is my least favorite but will last the longest meaning of course multi sessions over a couple days. With my 'married w/children' lifestyle, I haven't had more than a couple opportunities to try it like that but it does work.. Because it doesn't leave the system very quickly, the sides feel like a hangover the next day. Booze just makes it worse. Again that is just me, but it is cool to get wood several times over a couple days. I just don't like the 'hangover type' feeling I get from it. Works fast, long, and I prefer again the smaller dose of like 10mgs. of it. With alcohol, I really get a bad headache from it.
My favorite for my use is Viagra. I can mix it with the others, tailor the dose to my particular needs, and with proper usage, I get as had as a steel rod, and it feels like a pump from d.bol. It's almost like the d/bol of ed meds. When it's over you lose most of it. Fortunately the loss is only temporary..
I assume my results may not be typical, but just sharing my experiences..
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I agree about the insane sex and only have used any help for certain occasions. 
I'm cheap and that would have gotten to be expensive, with no refractory period. The best part was when we were leaving and she said, "Slow down aren't you sore?" I told her, "No I do this every weekend." Luckily we don't work together anymore and I don't return her calls either. 
