Coal tar is a dark, sticky liquid derived from wood and coal. It's used as an over-the-counter medicine to treat flaky skin conditions like psoriasis, seborrheic dermatitis (dandruff) and eczema. Because it contains salicylic acid, coal tar is a useful treatment for such skin conditions. But the remedy is also phototoxic and can further damage skin if exposed to sunlight.
The tar, which is generally found as a topical agent or an active ingredient in dandruff shampoos, remains active for around 24 hours [source: NPF]. It's recommended, though, that patients keep the treated areas of the skin (or scalp, in the case of a medicated shampoo) away from direct sunlight for 72 hours after use [source: Mayo Clinic].
Studies have shown that sunscreen can subvert the phototoxicity of coal tar [source: Diette et al].