Niagara Falls is a famous honeymoon destination. Viagra is a product to treat erectile dysfunction. A joke is:
   
Q: What’s the difference between your first and second honeymoon?
A: Niagara and Viagra.
 
The joke has been popular since at least July 1998, when it was published in Reader’s Digest magazine.
   
       
Wikipedia: Niagara Falls
Niagara Falls is the collective name for three waterfalls that straddle the international border between the Canadian province of Ontario and the American state of New York. They form the southern end of the Niagara Gorge.
 
From largest to smallest, the three waterfalls are the Horseshoe Falls, the American Falls and the Bridal Veil Falls. The Horseshoe Falls lies on the border of the United States and Canada with the American Falls entirely on the United States’ side, separated by Goat Island. The smaller Bridal Veil Falls are also on the United States’ side, separated from the American Falls by Luna Island.
 
Wikipedia: Sildenafil
Sildenafil, sold as the brand name Viagra among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. Its effectiveness for treating sexual dysfunction in women has not been demonstrated.
         
24 July 1998, Los Angeles (CA) Times, “Laugh Lines; Punch Lines,” pg. E2, col. 2:
Happy Anniversary: “Second honeymoons are now in vogue. On my first one, it was Niagara. Now, 25 years later, it’s Viagra.” (Charles J. Chop)
 
Google Books
The Reader’s Digest
Volume 153
Pg. 77:
QUESTION:  “What’s the difference between your first honeymoon and your second?”
Answer:  “The first, Niagara; the second, Viagra.”
- Contributed by David Letendre
 
Google Groups: soc.culture.vietnamese
Laughter, The Best Medicine
Nguyenzzz
7/29/98
(...)
Question:  “What’s the difference between your first honeymoon and your second?”
Answer:  “The first, Niagara; the second, Viagra.”
- Contributed by David Letendre
       
30 July 1998, Indianapolis (IN) Star, “Figuring out the really big news is really anyone’s guess” by Dick Cady, pg. B1:
And we’ve all heard all the Viagra jokes we can stand. (Sample: first honeymoon, Niagara; second honeymoon, Viagra.)
 
Google Groups: vietnet.vn.singles
[vn-singles] Fwd: Medicine
Hoang T Nguyen
3/1/99
(...)
>Question: “What’s the difference between your first honeymoon and your
>second?”
>Answer: 1.Niagara, 2. Viagra.
   
Google Books
The Girls from Winnetka
By Marcia Chellis
Bloomington, IN: iUniverse
2010
Pg. ?:
And when Viagra appeared on the market, he added that to the enjoyment of their relationship. He readily laughed with friends each time he said, “First honeymoon, Niagara. Second honeymoon, Viagra.”
 
Twitter
Kaloo
@Kaloo5
What’s the difference between the first honeymoon and the second?
First honeymoon, Niagara. Second honeymoon, Viagra
10:37 AM - 19 Jan 2012
   
Twitter
Kumbh Karan
@Ponga_Pandit
Banta: What’s the difference between the first & the second honeymoon?
Santa:The first honeymoon is Niagara the second is Viagra.
1:49 AM - 24 Mar 2012
   
Twitter
Mr. Women Seducer
@MrWomenSeducer
Q: What the difference between your first and second honeymoon?
A: Niagara and Viagra.
6:09 AM - 13 Sep 2014
   
Twitter
Darla a.k.a. Missy 🌺
@MNMissy
Q: What the difference between your first and second honeymoon?
A: Niagara and Viagra.
10:14 AM - 30 Sep 2014
 
Twitter
lorraine Jo King
@LahLahBean
Q: What the difference between your first and second honeymoon?
A: Niagara and Viagra.
1:06 PM - 19 Jul 2018