When it comes to using human toiletries on your pooch, the quick answer is "Don't": but this isn't the entire story. Human soaps and shampoos aren't recommended for your furry friend because of dogs' higher skin pH levels, but a few human-oriented products are safe for Fido's delicate complexion.
By far the most common dog-safe human soap is castile. Castile soaps are liquid, detergent-free, plant-oil-based soaps. What's the difference between a detergent and a soap? Detergents contain surfactants—sudsing agents that break down natural oils on the skin and hair. They're usually manufactured from coconut oil. Most commercial human soaps and shampoos contain detergents. Castile soaps are made from surfactant-free vegetable oils like olive or almond. They have low sudsing and rinse dirt away without breaking down natural body oils. These soaps kill fleas and ticks (they're also used on plants to kill insect pests). They're available in a variety of natural plant scents and are very gentle and nontoxic. Most of them should be diluted prior to use, so check the label on the bottle before you apply one to your pooch.
Though unscented glycerin soap base is sometimes called "casting soap," don't confuse this with castile. Glycerin soaps are solid, translucent and commercially available in a wide variety of colors and scents. Glycerin can be made from animal fat, plant oils, or synthetics, and glycerin soaps may contain detergents or other items that aren't good for your animal friend, so check the source and ingredients before using it on your pup. All-natural, plant-based, unscented glycerin bars, or ones scented with dog-safe botanical oils, are safest for your pet. Like castile soap, pure glycerin soaps are low-sudsing and don't strip the natural oils from your pet's coat.
Ye olde pine tar—the most old-fashioned and, er, fragrant soap around—is also one of the most highly touted for dog bath care. Pine tar soap is recommended by groomers and dog enthusiasts if your pup has hot spots. It's also recommended for skin conditions that don't respond well to medication. Most pine tar soaps are handmade with all-natural ingredients and don't disrupt doggy pH, but always check the label for detergents and other questionable items. There are also a number of brands specifically formulated with your furry best friend in mind.
As with all things interesting and smelly, it's best to keep soaps out of your pup's reach when bath time is over. All these soaps are nontoxic, but that doesn't mean they're good for your pooch to eat, or safe when ingested in large quantities. Eating castile soap will invariably cause your pup some messy intestinal distress, and the essential oils used to scent it can be harmful or poisonous when ingested. Glycerin isn't just for soap—it's also used to make laxatives. Eating a bar of it will have a distinct effect on Fido. Pine tar soap may contain small amounts of poisonous creosote and lye. In summary, wash to your heart's content, but secure your yummy soaps in a non-dog-accessible location.