It's not going to make much difference.
The tension differential is less than 5% overall.
If you can manage it, get another guitar and have each in the tuning you need.
One always has at least one guitar fewer than one already needs!
music.stackexchange.com/users/2808/Tim
Switching between standard E tuning and drop D tuning is so common that some guitarists use special tuning knobs that flip between the two with a switch.
I have a bass guitar version on my Fender Jazz bass.
Some guitarists have practiced this retuning so extensively that they can do it by feel with just the regular knob – if you have ever watched a medley of popular riffs on YouTube, you may have seen the performer do this.
In short, don’t worry about it!
music.stackexchange.com/users/6556/Bradd Szonye
I would say only when you actually tune up do you risk hurting the guitar but who does that anyway?
Drop tuning may affect the intonation of the guitar but that is in no way breaking the guitar, and can be fixed relatively easily.
music.stackexchange.com/users/7306/Neil Meyer
Not at all, I've used many, many guitars throughout the years.
All of which could hold tone, and had relatively decent intonation.
I would (and still do) switch between tunings.
Standard to Drop D, Drop D to Drop C#, back up to Standard.
Now it can overtime give your tuning pegs more wear and tear, but thats the nature of playing guitar.
Also changing and using alternate tunings is a great way to learn more chord structures, and is great practice for those who like to focus on progressions.
music.stackexchange.com/users/36796/andy_5860
I would also add to the very good answers, that in the past string tension was unfairly blamed for issues with humidity.
Guitars are often damaged because of dryness (like winter weather or desert heat, or simply a dry house environment) or proximity to heat.
The issues this low humidity causes falsely makes the strings look like the perpetrator.
And of course, if you try hard enough, with a weird enough tuning you can cause damage.
For example if you fully loosen the four bottom strings, and tighten the top two strings to an extreme and leave the guitar that way for weeks or months...
well then maybe you could cause the neck might to develop a twist.
music.stackexchange.com/users/15535/amalgamate
No problem.
The drop-d tuning only changes the total tension about four percent, so it will not do any damage to the guitar.
Your low E string will not last as long, however.
music.stackexchange.com/users/26744/Scott Wallace
I'm currently learning a song which needs drop-d tuning.    Since I found myself switching back and forth from drop-d to standard tuning a couple of times a day, I wonder if I'm risking damage to the guitar by doing so.   [It's an acoustic guitar, in case that matters]
Do I risk ruining my guitar by switching the tuning often?
music.stackexchange.com
<guitar><tuning><alternative-tunings>
music.stackexchange.com/questions/53202
music.stackexchange.com/users/None/
Guitars are often damaged because of dryness (like winter weather or desert heat, or simply a dry house environment) or proximity to heat. The issues this low humidity causes falsely makes the strings look like the perpetrator.  However, unless you indulge in a lot of weird tuning, you are unlikely to damage the guitar in that way. Guitars are often damaged because of dryness (like winter weather or desert heat, or simply a dry house environment) or proximity to heat.  With a weird tuning you can also cause neck problems.
Guitars are often damaged because of dryness (like winter weather or desert heat, or simply a dry house environment) or proximity to heat. Low humidity makes the strings look like the perpetrator. With a weird tuning you can also cause neck problems. For example, tighten the top two strings to an extreme and leave them loose for weeks or months.