No difference. They work the same. The name is different because they are made by different companies.
Different timing. R is usually given about 1/2 hr prior to a meal. No longer the short acting Insulin of choice because others more effective having shorter 1/2 lives. Nph may be combined with short acting Insulin or given alone to provide later coverage. Starts to peak in ~ 4 hrs, though effect will last for several. It peaks which predisposes people to low sugars. Newer insulins replace nph for better control.
Similar. Humulin and novolin are made by two different companies, but they make similar products, so Humulin R would be similar to novolin r, Humulin N would be similar to novolin n. These are all "old fashion" insulins and not used very much anymore compared to humalog, novolog, lantus, Levemir (insulin detemir) insulins.
Should be. . . Novolin (insulin) n and Humulin N are just two different brands of Insulin nph w/onset in 1-2hrs, peak in 4-14hrs & duration of 10->24hrs. Therefore, your body might notice some differences even though theoretically they're the same. Ask your family doc or endocrinologist before switching. S/he may want to change the frequency of your self-checks. Ps if you're ever unsure, ask your doctor!
Yes. You are basically switching from nph to another nph. They are just made by a different company. The onset of action and respond should be the same.
My doctor wants me to take humulin with novolin twice a day plus zyrtec (cetirizine) becsuse im allergic to insulin and sulfa. Is this safe?
Your doctor. Has determined that the benefits of this treatment outweighs its risks. Every medication has side-effects, and every medical treatment has risk. It is best to follow your doctor's advice.