A lot of the time when doing research online (i.e., Googling), the most common results for growing hair will be for the scalp. Even when searching with the beard keyword, many people that post articles will post techniques that work for the scalp, working with the false assumption that "hair is hair". We should not always use the same methods of growing hair on our faces as we do our scalps.
In fact, be extra cautious about using certain products on your face that are meant for your scalp; even if it's branded with the word beard, many times these are simply marketed as such but still use the same general ingredients as scalp products. Maybe less harsh as to not cause disturbance on a sensitive face, but look at the ingredients before grabbing these beard-growing oils and products.
Avoid: Essential Oils with DHT Blockers
On the beard forum, we have a topic that lists some of the ingredients that have 5-alpha reductase inhibitors. Some of the most popular ones can be found in many beard oils, of which many claim to help with beard growth.
Dihydrotestosterone (DHT), is needed when growing androgenic hair, which is body hair: excluding the hair on our scalp, but including beards. 5-alpha reductase (5AR) is the enzyme that converts testosterone into DHT, and blocking this enzyme from doing its job is only ever helpful when it comes to those that are balding from something such as male pattern baldness. If you're prone to genetic baldness, the last thing you want is more DHT as it can accelerate the rate of balding. But here's the thing: those predisposed to balding will lose their head hair even at normal levels of DHT.
To learn more about DHT, see Understanding the Importance of DHT for Health and Beard Growth.
List of Essential Oils with DHT Blockers
This list will need to be updated as we discover more about each essential oil. For the sake of keeping the article straightforward, the sources can be found in the forum topic linked above.
Some of the oils listed are plant-derived antiandrogens. Androgens are considered the male hormone (also found somewhat in females; just as estrogen, the female hormone, is found in males). A beard requires male hormones, as it's a secondary sex characteristic — which is a reason as to why most females cannot grow beards without some type of steroid use.
While estrogen levels are significantly lower in males compared to females, estrogens nevertheless also have important physiological roles in males - Wikipedia
Where 5AR Inhibitors (DHT Blockers) are Found?
5AR inhibitors are typically found in hair and skincare products. Usually, but not exclusively, in products that are meant to help head hair growth or baldness prevention, and also more typically in women's products than in men's.
Some brands of minoxidil, a popular medication now used topically to regrow hair and prevent further hair loss, contain DHT blockers. Because there are no brands of minoxidil being sold that are targeted toward beard growth (in the United States), and because it's not FDA approved for anywhere except for the vertex (circular area) of the head, DHT blockers can be very common in certain mixtures as it's sold for head hair; not beards. If you're one of the guys on the minoxidil journey for a beard, that's something to keep in mind when purchasing your minox. These brands will normally state DHT blockers on the labels, however, as it tends to be a point of pride.
Note: minoxidil not being FDA approved for anywhere other than the vertex is simply due to scientific studies not being conducted elsewhere on the body with it. Not being FDA approved does not mean that minoxidil will be ineffective for other places around the body or for receding hair lines. It simply cannot be marketed that way in the United States.
Lotions, Soaps, & Shampoos
As mentioned above, 5AR inhibitors might be found in hair and skin products, usually as part of the fragrance in the form of essential oils. The essential oils listed a few paragraphs up is not an exhaustive list, but it does have some of the most popular oils in it. Check the ingredient labels on your products, and check them before purchasing to save a little money.
Sometimes a label might list the ingredients and then add a catch-all such as "and essential oils for smell." If you come across this, email the company and ask if they have specific DHT-inhibiting oils in their product. Usually, "fragrance" or "flavor" will be found on the label, which will either be chemicals, essential oils, or a mixture of these two things. Because certain mixtures can fall under being a trade secret, companies can "hide" what the fragrance is, but email them anyway and ask that same question.
“Essential Oils” and “Aromatherapy”
There is no regulatory definition for “essential oils,” although people commonly use the term to refer to certain oils extracted from plants. The law treats Ingredients from plants the same as those from any other source.
For example, “essential oils” are commonly used in so-called “aromatherapy” products. If an “aromatherapy” product is intended to treat or prevent disease, or to affect the structure or function of the body, it’s a drug. To learn more, see “Aromatherapy.”
Similarly, a massage oil intended to lubricate the skin is a cosmetic. But if claims are made that a massage oil relieves aches or relaxes muscles, apart from the action of the massage itself, it’s a drug, or possibly both a cosmetic and a drug.
Bear in mind that a lot of soaps are fine to use, particularly natural soaps; the skin's barrier is fairly resilient and it's unlikely that a soap will penetrate for the short amount of time that it's on the skin. Adding to that, when essential oils are used merely for scent at lower percentages, it's even more unlikely that it will hinder your beard. I would say that most soaps are fine to use, but if you're in doubt, simply opt for one without the ingredients that inhibit DHT production.
Shampoos and lotions tend to have more penetrating ingredients in them as opposed to soap, but this is not a hard and fast rule. Each and every product is different, so knowing your labels and ingredients is the best option you have to avoid unwanted antiandrogens and chemicals.
What Other "Scalp-Related" Things Should I Avoid?
Other than products, at the top of my head I'd say nothing. Techniques like improving blood circulation via massaging, cleaning your face of flakes and dirt and so on will all help to improve beard growth. It mostly comes down to products that are meant for the head and repurposed for the beard where people run into trouble.
Keep in mind that when you use products sparingly, things should be fine. It's usually when people go overboard when things start getting harmful (folks tend to do that when reaching for beard perfection). The same thing can be said about absolutely anything, including water. So basically, know what you're using and don't overdo it and things will be a-okay overall.
At the end of the day, consult your doctor if you're worried about anything and take all advice you find online with a grain of salt.