Men, you can skip this section since this won't really pertain to you, unless you're buying for someone else.
Generally there really isn't a difference between Japanese and American feminine hygiene products besides brand names and labeling. I will describe each feminine hygiene product and what would be best for your light and heavy days.
Pads in Japan are called ナプキン(napukin) or sanitary napkin. They are labeled according to size in cm and whether they have wings, hanetsuki (羽根つき）or not (羽なし）hanenashi.
There is a lot of variety in pads in Japan. There are long pads that can be worn at night during your heavy days (as pictured above) and pads that can be worn during your regular days. Pads can range from thin to really fluffy. If you can't read kanji the pictures on the packaging give you a good idea of what the pad looks like. Usually the packaging has a little picture of a sun or a moon to symbolize if they're for nighttime or daytime use.
If you're really concerned about how heavy your days are and need a specific pad here are some quick kanji. Underneath the picture of the sun on the right of the package is for what type of flow the pad is intended for.
多い日用 - heavy day
軽い日用 - light day
昼用 - day use
夜用 - night use
ふつう用 - regular day use
Panty liners are called ライナー (liner) or rainaa. These are used for your really light days, or for discharge. There really isn't a difference between liners in America and Japan, these will be labeled with having wings or no wings just like pads.
If you don't want to deal with the hassle of reading and remembering all this and you're staying for a short time in Japan I would suggest just bringing your own. When packing just take the pads out of the box and put them in a ziploc bag to save space.
Tampons in Japan are called タンポン (tampon) and are labeled as follows
レギュラー - regular
スーパー - super
スーパープラス - super plus
ライト - light
ソフト - soft
コンパクト - compact
Generally, there really isn't a difference between and American and Japanese tampons. The only real difference is the applicator type.
There are compact, regular and finger type applicators. Compact means that the applicators are small, regular means they are regular size applicators (about the same as America tampons) and finger applicators mean that there are no applicators. The package has a picture of what the tampon looks like so you can get a good idea of what it looks like if you can't remember kanji.
Alright, heres where it gets to the point where you might want to bring your own stuff. Japan has a weird selection of deodorant, they sell it, but it's really different than America.
Japanese people don't sweat a lot like westerners do, they have less sweat glands that produce smelly smells so they don't really need strong deodorants.
Deodorant types consist of water roll ons, powder sprays, and rarely powder roll ons that are similar to what we have here, but these generally cost around 8$ and are really tiny. I would suggest brining your own deodorant from home if you don't want to smell and spend 8$ for something that won't last a month.
For men the selection is also really small. However, Japan does sell a deontaulle crystal stone that works really well as a deodorant. However, this is also kinda expensive as far as deodorant goes and it would be better to bring something from home.
Shampoo and Conditioner
If you don't really care about what you use on your hair you will be fine in Japan in terms of shampoo and conditioner. They have some popular brands here , and some unique Japanese brands. The prices don't differ too much from America so it's not a big deal if you forget a travel size shampoo.
Japan also have bags full of shampoo and conditioner that is used to refill bottles after you run out.
Side Note: conditioner can either be labeled コンヂしょなー (conditioner) or リンス (rinse) and shampoo is labeled as シャンプー (shampoo)
So for most of your journey you can get away with buying a majority of products from Japan, but it's always better to bring things your body is used to if you can fit it into your suitcase. Happy traveling :)